# Interleaving Elements from Two Linked Lists in Python: Top 5 Methods

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π‘ Problem Formulation: We are aiming to interleave the elements from two different linked lists in Python. For example, having two linked lists `A->B->C` and `1->2->3`, we want to merge them into a single list `A->1->B->2->C->3`. This article discusses five different ways to accomplish this task, taking into account efficiency, readability, and versatility of the solutions provided.

## Method 1: Iterative Approach

The iterative approach to interleaving two linked lists involves traversing both lists simultaneously and relinking the nodes. This method is memory efficient because it works in-place without creating additional nodes or lists. The function specification would involve two linked list heads as input and the modification of the lists to produce the interleaved result.

Here’s an example:

```class ListNode:
def __init__(self, value=0, next=None):
self.val = value
self.next = next

def interleaveLists(l1, l2):
p1, p2 = l1, l2
while p1 and p2:
temp1, temp2 = p1.next, p2.next
p1.next, p2.next = p2, temp1
p1, p2 = temp1, temp2
return l1

# Example Lists
list1 = ListNode('A', ListNode('B', ListNode('C')))
list2 = ListNode('1', ListNode('2', ListNode('3')))

# Interleave Lists
interleaved_list = interleaveLists(list1, list2)
```

The output of this code will be the linked list `A->1->B->2->C->3` as a result of modifying the original lists.

This code defines a `ListNode` class and the `interleaveLists` function. It then creates two example lists and interleaves them by altering the `next` pointers of the nodes. The nodes are relinked in-place by alternating between the next elements of each list. This efficient approach ensures that no additional space is used, as the existing nodes are simply rearranged.

## Method 2: Recursive Approach

The recursive method for interleaving linked lists uses the call stack to handle the interweaving process. Each recursive call handles one pair of nodes from both lists, linking them appropriately before diving deeper. This is often seen as a more elegant solution, but it may consume more stack space for long lists.

Here’s an example:

```class ListNode:
def __init__(self, value=0, next=None):
self.val = value
self.next = next

def interleaveLists(l1, l2):
if not l1:
return l2
if not l2:
return l1
next1, next2 = l1.next, l2.next
l1.next = l2
l2.next = interleaveLists(next1, next2)
return l1

# Example usage
list1 = ListNode('A', ListNode('B', ListNode('C')))
list2 = ListNode('1', ListNode('2', ListNode('3')))
interleaved = interleaveLists(list1, list2)
```

The output will once again be an interleaved linked list: `A->1->B->2->C->3`.

This code snippet features a recursive version of the interleaving function. The base cases return the non-empty list when the other is empty. Each call to `interleaveLists` links two nodes and then proceeds to interleave the rest of the lists. The elegance of recursive solutions comes with the overhead of additional space used on the call stack, which can be a downside for very long lists.

## Method 3: Using Auxiliary Data Structure

Using an auxiliary data structure like a stack or a queue can help simplify the interleaving process. We can store pointers to the nodes of both lists in a queue and then pop them alternately to create the interleaved list. This method simplifies the code and can be easier to understand at a glance, but it requires extra memory proportional to the combined lengths of the two lists.

Here’s an example:

```from collections import deque

class ListNode:
def __init__(self, value=0, next=None):
self.val = value
self.next = next

def interleaveLists(l1, l2):
q = deque()
p1, p2 = l1, l2
while p1 or p2:
if p1:
q.append(p1)
p1 = p1.next
if p2:
q.append(p2)
p2 = p2.next
dummy = ListNode()
cur = dummy
while q:
cur.next = q.popleft()
cur = cur.next
cur.next = None
return dummy.next

# Example usage
list1 = ListNode('A', ListNode('B', ListNode('C')))
list2 = ListNode('1', ListNode('2', ListNode('3')))
interleaved = interleaveLists(list1, list2)
```

The output is an interleaved list: `A->1->B->2->C->3`.

In this method, we create a `deque` from the `collections` module to act as a double-ended queue. We then iterate over both lists, adding all their nodes to the queue. Finally, we create a new list by popping nodes from the queue and linking them one by one. While this avoids manual pointer manipulation, the extra space used for the queue can be seen as a trade-off.

## Method 4: Using Generators

Python’s generators provide a lazy way to iterate over elements one at a time. We can create generator functions for both linked lists that yield their elements. Then, we can use the `zip` function to create a paired iterator and link the nodes in interleaved fashion. This method reduces the complexity of the function and can handle lists of different lengths elegantly. However, it may not be as efficient as the in-place methods for very long lists.

Here’s an example:

```class ListNode:
def __init__(self, value=0, next=None):
self.val = value
self.next = next

def listGenerator(node):
while node:
yield node
node = node.next

def interleaveLists(l1, l2):
gen1, gen2 = listGenerator(l1), listGenerator(l2)
dummy = ListNode()
current = dummy
for n1, n2 in zip(gen1, gen2):
current.next, current = n1, n1
current.next, current = n2, n2
current.next = next(gen1, None) or next(gen2, None)
return dummy.next

# Example usage
list1 = ListNode('A', ListNode('B', ListNode('C')))
list2 = ListNode('1', ListNode('2', ListNode('3')))
interleaved = interleaveLists(list1, list2)
```

The output will be: `A->1->B->2->C->3`.

The provided code defines two generators that yield nodes from the linked lists. These generators are used with `zip` in a loop to perform the interleaving process. The `next` function ensures that if one list is longer than the other, the remaining nodes will be appended. Generators offer a clean approach, but the performance might not be optimal compared to in-place manipulation.

## Bonus One-Liner Method 5: Using List Comprehensions and Unpacking

Pythonβs list comprehension and unpacking (*) features allow us to condense the interleaving process into a single line of code. While this one-liner approach can be seen as Pythonic and concise, it assumes that our linked lists have been converted to Python lists and will only interleave up to the shorter list’s length, omitting any remaining elements from the longer list.

Here’s an example:

```def interleaveLists(l1, l2):
return [val for pair in zip(l1, l2) for val in pair]

# Assuming l1 and l2 are now Python lists
list1 = ['A', 'B', 'C']
list2 = ['1', '2', '3']
interleaved = interleaveLists(list1, list2)
```

The output is a Python list: `['A', '1', 'B', '2', 'C', '3']`.

This concise snippet interleaves the elements of the two Python lists using list comprehension and the `zip` function. It flattens the paired elements produced by `zip` into a single list. As such, it is important to convert linked lists into Python lists before using this method, and it is not suitable for conserving the original linked list structure.

## Summary/Discussion

• Method 1: Iterative Approach. Efficient and in-place. May be complex to understand.
• Method 2: Recursive Approach. Elegant and straightforward. Potentially high stack usage.
• Method 3: Auxiliary Data Structure. Simplifies code. Requires extra space for queue.
• Method 4: Using Generators. Reduces complexity and good for different list lengths. Lower performance for very long lists.
• Method 5: One-Liner List Comprehensions. Pythonic and concise. Requires list conversion and only suitable for equal length lists.