Solve the Medium-level Reorder List coding interview problem online in Python & JavaScript. Run your code against real test cases in the browser and study worked examples. It covers linked list, two pointers, and stack. Read the step-by-step solution and complexity analysis. Java starter code is also included for reference (no in-browser JVM yet, so it can't be run here).

Reorder List – Solve Online in Python & JavaScript

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?

## Execution note
Java/Go/Rust don't support this problem's ListNode/TreeNode structure yet — the starter is reference-only. Try Python or JavaScript to run and submit.

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.