Remove Nth Node From End of List

Add Two Numbers

Delete the n-th node from the end of a singly linked list in one pass.

Given the `head` of a linked list, remove the `n`-th node from the end of the list and return its head.

**Input / output**
- Input: `head: ListNode` (JSON test fixture is a plain array of node values), `n: int`
- Output: `ListNode` — the head of the list after removal (serialized the same way)

## Example
```json
Input: {"head":[1,2,3,4,5],"n":2}
Output: [1,2,3,5]
```
(The 2nd node from the end, value `4`, is removed.)

## Constraints
- The number of nodes in the list is between 1 and 30.
- `0 <= node value <= 100`
- `1 <= n <= number of nodes` (n is always valid, so the removed node always exists)

## Follow-up
Can you do this in a single pass, without first counting the length of the list?

The key pattern is: Two-pointer gap technique: advance a lead pointer n steps ahead, then move both until the lead hits the end. 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(L) where L is the list length. Space: O(1) auxiliary in every language. The Rust implementation uses two passes (one pass to count length, one to remove the node) because the classic one-pass pointer-gap technique runs into move/borrow conflicts on Option<Box<ListNode>>; the other implementations use the one-pass pointer-gap technique.

All 5 languages below run and submit against the remote judge for this problem — Java/Go/Rust use the same real ListNode structural-type support the judge added for the rest of the linked-list/tree track.