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:**

![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg)

**Input:** head = [1,2,3,4,5], k = 2

**Output:** [2,1,4,3,5] 

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg)

**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?