Reverse Nodes in k-Group

Question

## Problem

Given a linked list, reverse the nodes `k` at a time and return the modified list; leftover tail (< k) stays as is.

## Example
```
1->2->3->4->5, k=2 => 2->1->4->3->5
```

## Constraints
- 1 ≤ k ≤ n ≤ 5000

## Approach
In-place reversal per group; careful pointer bookkeeping between groups.