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 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] ```   **Constraints:** - The number of nodes in the list is `n`. - `1 <= k <= n <= 5000` - `0 <= Node.val <= 1000`   **Follow-up:** Can you solve the problem in `O(1)` extra memory space?