# 5 Best Ways to Find the Largest Sum Value of a BST in a Given Binary Tree in Python

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π‘ Problem Formulation: Given a binary tree, the task is to find the maximum sum of all values in any subtree that is also a Binary Search Tree (BST). The binary tree is a fundamental data structure consisting of nodes, where each node contains a value, and references to left and right child nodes. The goal is to identify a valid BST within the binary tree and calculate the sum of its elements, aiming for the largest possible sum. For instance, given an input binary tree, the largest sum value of a BST within it may be “x”, which is what the program should output.

## Method 1: Recursive Traversal and Validation

This method involves performing a recursive depth-first traversal of the binary tree. At each node, the algorithm checks if the subtree rooted at that node is a valid BST and, if so, calculates its sum. The maximum sum encountered during the traversal is tracked, thereby finding the largest BST sum in the tree.

Here’s an example:

```def largest_bst_sum(root):
max_sum = [0]

def is_bst(node, low, high):
if not node:
return 0, True
if not low < node.value < high:
return float('-inf'), False
left_sum, is_left_bst = is_bst(node.left, low, node.value)
right_sum, is_right_bst = is_bst(node.right, node.value, high)
if is_left_bst and is_right_bst:
return left_sum + node.value + right_sum, True
return float('-inf'), False

def find_max_bst_sum(node):
if not node:
return
sum, is_bst = is_bst(node, float('-inf'), float('inf'))
if is_bst:
max_sum[0] = max(max_sum[0], sum)
find_max_bst_sum(node.left)
find_max_bst_sum(node.right)

find_max_bst_sum(root)
return max_sum[0]

# Assume we have a binary tree with a BST inside it
# Example usage:
# largest_bst_sum(root)
```

Output would be the highest sum of any subtree within the binary tree that is a valid BST.

The code snippet defines a recursive helper function `is_bst` that checks whether a subtree is a valid BST and calculates its sum. It also defines `find_max_bst_sum`, which traverses the binary tree in search of the largest BST sum, updating the `max_sum` variable accordingly.

## Method 2: Post-order Traversal with Subtree Tracking

This technique uses post-order traversal of the tree. Instead of validating the entire subtree at every node, this method keeps track of the subtrees’ range and whether they are valid BSTs. It returns this information along with the subtree sum up the tree, thereby avoiding redundant validations and improving efficiency.

Here’s an example:

```def find_largest_bst_sum(root):
largest_sum = [0]

def post_order(node):
if not node:
return float('inf'), float('-inf'), 0, True
l_min, l_max, l_sum, l_bst = post_order(node.left)
r_min, r_max, r_sum, r_bst = post_order(node.right)
if l_bst and r_bst and l_max < node.value < r_min:
curr_sum = l_sum + r_sum + node.value
largest_sum[0] = max(largest_sum[0], curr_sum)
return min(node.value, l_min), max(node.value, r_max), curr_sum, True
return 0, 0, 0, False

post_order(root)
return largest_sum[0]

# Example usage:
# find_largest_bst_sum(root)
```

Here, the maximum sum achieved within any subtree that is a valid BST will be returned.

The function `post_order` returns a tuple containing the minimum and maximum values of a subtree, its sum, and a boolean indicating whether it’s a valid BST. This information helps to check for the BST property without repeated computation. The largest sum is updated accordingly in the `largest_sum` variable.

## Method 3: Dynamic Programming with Subtree Data

Dynamic programming can be applied to this problem by caching the results of subtree evaluations. Each node stores data about the maximum/minimum value of its subtree, the sum, and its validity as a BST. This information is then used to compute larger BST sums without redundant subtree checks.

Here’s an example:

```class NodeInfo:
def __init__(self, min_val, max_val, sum_val, is_bst):
self.min_val = min_val
self.max_val = max_val
self.sum_val = sum_val
self.is_bst = is_bst

def find_max_sum_as_bst(root):
max_sum = [0]

def helper(node):
if not node:
return NodeInfo(float('inf'), float('-inf'), 0, True)
left = helper(node.left)
right = helper(node.right)
if left.is_bst and right.is_bst and left.max_val < node.value < right.min_val:
node_sum = left.sum_val + right.sum_val + node.value
max_sum[0] = max(max_sum[0], node_sum)
return NodeInfo(min(node.value, left.min_val), max(node.value, right.max_val), node_sum, True)
return NodeInfo(0, 0, 0, False)

helper(root)
return max_sum[0]

# Example usage:
# find_max_sum_as_bst(root)
```

The result should be the largest sum from all BSTs subtrees present in the binary tree.

The `helper` function computes the node information for every node in the tree, while `NodeInfo` class is used to store subtree information. This method reduces time complexity significantly as compared to previous methods.

## Method 4: Optimized Space Usage with In-Place Tracking

For space optimization, the same dynamic programming approach can be used, but instead of creating a class, a tuple is used to return multiple values from the helper functions. This reduces the overhead of object creation and provides a slightly more space-efficient solution.

Here’s an example:

```def find_largest_bst_sum(root):
max_sum = [0]

def helper(node):
if not node:
return float('inf'), float('-inf'), 0, True
l_min, l_max, l_sum, l_bst = helper(node.left)
r_min, r_max, r_sum, r_bst = helper(node.right)
if l_bst and r_bst and l_max < node.value < r_min:
curr_sum = l_sum + r_sum + node.value
max_sum[0] = max(max_sum[0], curr_sum)
return min(node.value, l_min), max(node.value, r_max), curr_sum, True
return 0, 0, 0, False

helper(root)
return max_sum[0]

# Example usage:
# find_largest_bst_sum(root)
```

The function will output the largest BST sum found in the given binary tree.

The `helper` function leverages tuples to handle multiple outputs which inform about the BST validity and its sum. While similar to Method 2, this approach is more focused on space efficiency, sacrificing readability for reduced space usage.

## Bonus One-Liner Method 5: Functional Programming with Immutable Data

In this elegant one-liner method, we embrace the power of functional programming. We define a lambda function that evaluates and returns the relevant information about the binary tree’s validity as a BST and updates the sum in a functional, stateless manner. This method is more of a theoretical interest and serves to show the flexibility of Python’s functional capabilities.

Here’s an example:

```# This one-liner is more of a conceptual representation and might not be as performant or readable.
max_bst_sum = lambda root: max((lambda f: f(f, root, float('-inf'), float('inf'), [0])) (lambda self, node, lo, hi, res: res[0] if not node else max(res[0], self(self, node.left, lo, node.value, res), self(self, node.right, node.value, hi, res)) if lo < node.value < hi else 0) if root else 0)

# Example usage:
# max_bst_sum(root)
```

If a subtree rooted at the given binary tree is a BST, the maximum sum will be returned.

This one-liner defines a recursive lambda function wrapped in another lambda to maintain the function reference and the largest sum ‘res’. This code is compact but comes at the cost of readability and practical complexity.

## Summary/Discussion

• Method 1: Recursive Traversal and Validation. Strengths: Simple and straightforward. Weaknesses: Potentially high time complexity due to repeated subtree validation.
• Method 2: Post-order Traversal with Subtree Tracking. Strengths: More efficient than Method 1 by avoiding redundant validations. Weaknesses: Increased space complexity due to additional variables.
• Method 3: Dynamic Programming with Subtree Data. Strengths: Highly efficient with memoization. Weaknesses: Increased complexity, which may impact readability and maintenance.
• Method 4: Optimized Space Usage with In-Place Tracking. Strengths: More space-efficient than Method 3. Weaknesses: Less readable due to tuple packing/unpacking.
• Method 5: One-Liner Functional Programming. Strengths: Compact and shows the versatility of functional programming in Python. Weaknesses: Difficult to understand and debug due to its compactness and recursion complexity.
Please note that for a real world project, using functional programming and especially in a one-liner form as in Method 5, could be counterproductive due to complexity and maintainability issues. These methods have been included to showcase the range of approaches to solve the problem.