Reorder List

Clone Graph

Linked List Cycle

Reorder a linked list by alternating nodes from its front and back.

Reorder `L0 → L1 → … → Ln` into `L0 → Ln → L1 → Ln-1 → …` and return the head.

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

## Constraints
- 1 <= number of nodes <= 50000
- 1 <= node value <= 1000

## Follow-up
Can you achieve O(1) auxiliary space by reversing the second half?

The key pattern is: Split, reverse the second half, then weave halves. 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(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.