Problem: Reverse Nodes in k-Group

Statement

Given the head of a singly linked list and an integer k, reverse the nodes of the list in groups of k consecutive nodes at a time, and return the modified list.

k is a positive integer that does not exceed the length of the linked list. If the total number of nodes is not evenly divisible by k, the remaining nodes at the end of the list should be left in their original order.

You may not modify the values stored in the nodes. Only the node connections themselves may be rearranged.

Note: Can you solve the problem using $O(1)$ extra memory space?

Constraints:

• The number of nodes in the list is n.

• $1 \leq$ k $\leq$ ...