Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

k is a positive integer ≤ length of the linked list. If the number of nodes is not a multiple of k, the left-over nodes at the end should remain as-is.

You may not alter the values in the list's nodes — only the 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]

Constraints:

• Number of nodes: n
• 1 <= k <= n <= 5000
• 0 <= Node.val <= 1000

Follow-up: Can you solve this in O(1) extra memory?