public class Solution
extends Object
25 - Reverse Nodes in k-Group\.
Hard
Given a linked list, reverse the nodes of a linked list _k_ at a time and return its modified list.
_k_ is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of _k_ then left-out nodes, in the end, should remain as it is.
You may not alter the values in the list's nodes, only nodes themselves may be changed.
**Example 1:**
**Input:** head = [1,2,3,4,5], k = 2
**Output:** [2,1,4,3,5]
**Example 2:**
**Input:** head = [1,2,3,4,5], k = 3
**Output:** [3,2,1,4,5]
**Example 3:**
**Input:** head = [1,2,3,4,5], k = 1
**Output:** [1,2,3,4,5]
**Example 4:**
**Input:** head = [1], k = 1
**Output:** [1]
**Constraints:**
* The number of nodes in the list is in the range `sz`.
* `1 <= sz <= 5000`
* `0 <= Node.val <= 1000`
* `1 <= k <= sz`
**Follow-up:** Can you solve the problem in O(1) extra memory space?