Merge Two Sorted Linked Lists

Problem Statement

Given the heads of two sorted linked lists, merge them into one sorted linked list and return the head of the merged list.

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Example

Input
List1
1 -> 2 -> 4

List2
1 -> 3 -> 4

Output
1 -> 1 -> 2 -> 3 -> 4 -> 4

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Approach Iterative Method

Compare nodes from both lists and build a new sorted list.

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Steps

1 Create a dummy node to store result
2 Compare values of both lists
3 Attach smaller node to result
4 Move pointer of that list
5 Repeat until one list becomes empty
6 Attach remaining nodes

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Code

```python id="mll1"
class ListNode:
def init(self, val=0, next=None):
self.val = val
self.next = next

def mergeTwoLists(l1, l2):
dummy = ListNode(0)
tail = dummy

while l1 and l2:
    if l1.val < l2.val:
        tail.next = l1
        l1 = l1.next
    else:
        tail.next = l2
        l2 = l2.next
    tail = tail.next

if l1:
    tail.next = l1
if l2:
    tail.next = l2

return dummy.next

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## Approach Recursive Method

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### Code



```python id="mll2"
def mergeTwoLists(l1, l2):
    if not l1:
        return l2
    if not l2:
        return l1

    if l1.val < l2.val:
        l1.next = mergeTwoLists(l1.next, l2)
        return l1
    else:
        l2.next = mergeTwoLists(l1, l2.next)
        return l2

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Explanation

The iterative method builds the merged list step by step using a dummy node. The recursive method compares nodes and builds the list during function calls.

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Expected Output

Input
1 -> 2 -> 4
1 -> 3 -> 4

Output
1 -> 1 -> 2 -> 3 -> 4 -> 4

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Conclusion

Merging two sorted linked lists is a basic and important problem. It helps in understanding linked list traversal and pointer manipulation.

Practice both iterative and recursive methods to improve your coding skills.