# 5 Best Ways to Implement Depth First Search on Binary Trees Using Python Recursion

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π‘ Problem Formulation: Depth First Search (DFS) is a fundamental algorithm used to traverse or search through the elements of a binary tree. It’s essential in solving problems that require visiting every node of a tree, such as checking for the existence of a value or finding the path to a specific node. The goal is to create a Python program that can perform a DFS on a binary tree using recursion, with an input of the root node and a target value, and the desired output being a boolean indicating whether the target value exists within the tree.

## Method 1: Preorder Traversal

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.

Preorder traversal is a classic way to perform DFS on binary trees. It involves visiting the current node before its child nodes, moving forward recursively through the left subtree, and then the right subtree.

Here’s an example:

```class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None

def preorder_search(root, target):
if root is None:
return False
if root.val == target:
return True
return preorder_search(root.left, target) or preorder_search(root.right, target)

# Example tree construction
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)

print(preorder_search(root, 2))
```

The output of this code snippet:

`True`

This method structures the DFS in a depth-first manner, starting at the root. The `preorder_search` function takes a tree node and a target value, checking if the current node matches the target. If not, it proceeds with the left child followed by the right child using recursion, stopping and returning `True` when the target is found, and `False` otherwise.

## Method 2: Inorder Traversal

In an inorder traversal, the left subtree is explored first, followed by the current node, and finally the right subtree. This method of traversal can be useful especially for binary search trees, where it retrieves nodes in a sorted order.

Here’s an example:

```def inorder_search(root, target):
if root is None:
return False
if inorder_search(root.left, target):
return True
if root.val == target:
return True
return inorder_search(root.right, target)

print(inorder_search(root, 2))
```

The output of this code snippet:

`True`

In this example, the `inorder_search` function visits nodes in an inorder fashion. It starts with the left-most node, ascending to each parent, and then descending into the right subtree. The search terminates once the target is found returning `True`, or upon reaching the end of the tree without success, returning `False`.

## Method 3: Postorder Traversal

This method involves recursively exploring the left and right subtrees before processing the current node. It’s useful to post-process the subtrees, for instance, when we need to delete nodes or check subtree properties.

Here’s an example:

```def postorder_search(root, target):
if root is None:
return False
if postorder_search(root.left, target) or postorder_search(root.right, target):
return True
return root.val == target

print(postorder_search(root, 3))
```

The output of this code snippet:

`True`

Through the `postorder_search` function, each node is processed after its subtrees. If the target is found in either subtree, the function returns `True`. If it gets to a leaf and hasn’t found the target, the node’s value is compared to the target, and the result is returned.

## Method 4: Iterative Deepening DFS

Iterative Deepening DFS combines the space efficiency of DFS with the level-order search properties of Breadth-First Search (BFS). The tree is traversed in depth-bound increments, like a series of expanding DFS limited searches.

Here’s an example:

```def DLS(root, target, depth):
if root is None:
return False
if depth == 0 and root.val == target:
return True
if depth > 0:
return DLS(root.left, target, depth-1) or DLS(root.right, target, depth-1)
return False

def IDDFS(root, target, max_depth):
for depth in range(max_depth):
if DLS(root, target, depth):
return True
return False

print(IDDFS(root, 3, 3))
```

The output of this code snippet:

`True`

In this approach, `IDDFS` function calls the `DLS` (depth-limited search) for increasing depths. If `DLS` finds the target within the depth limit, it returns `True`. This method ensures complete coverage of nodes up to the maximum depth specified.

## Bonus One-Liner Method 5: Lambda Expression

In Python, we can compact the DFS logic using a lambda expression to achieve the same functionality in a concise form.

Here’s an example:

```dfs = lambda node, target: node and (node.val == target or dfs(node.left, target) or dfs(node.right, target))

print(dfs(root, 4))
```

The output of this code snippet:

`True`

The lambda function `dfs` encapsulates the depth-first search by evaluating the current node’s value or the search results of the left and right subtrees through short-circuit evaluation. This one-liner elegantly demonstrates the power and simplicity that can be achieved with Python’s lambda expressions.

## Summary/Discussion

• Method 1: Preorder Traversal. Easy to understand. Visits nodes in “root-left-right” order. It’s not sorted, which might be a drawback for certain applications.
• Method 2: Inorder Traversal. Retrieves nodes in non-decreasing order for binary search trees. Less intuitive for general binary trees that are not BSTs.
• Method 3: Postorder Traversal. Useful for subtree postprocessing tasks. Can be less intuitive due to late evaluation of the root node.
• Method 4: Iterative Deepening DFS. Combines BFS level-wise benefits with DFS’s lower memory footprint. More complex to implement. Can be slower due to repeated visits.
• Bonus Method 5: Lambda Expression. Extremely concise. Less readable for those unfamiliar with lambda expressions or recursion.