Merge Two Sorted Lists

Linked List Cycle

Lowest Common Ancestor of a BST

Merge two sorted linked lists by rewiring their nodes.

Merge two ascending linked lists and return the head of one sorted node chain.

## Example
```json
Input: {"list1":[1,2,4],"list2":[1,3,4]}
Output: [1,1,2,3,4,4]
```

## Constraints
- 0 <= total nodes <= 100
- -100 <= node value <= 100
- Both lists are sorted

## Follow-up
Compare iterative O(1)-space merging with recursive merging.

The key pattern is: Two pointers splice the smaller next node. Identify the state and invariant before coding.

Recognize the Pattern

Advance one state transition at a time. Mark or update state before exploring dependent work.

Build the Algorithm

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.

Check Edge Cases

Time: O(m + n). Space: O(1).

Java/Go/Rust below are reference solutions for learning the pattern in those languages; only Python and JavaScript can be run and submitted for this problem, since the remote judge doesn't support the ListNode structure yet.