Reverse Nodes in k-Group

The task is to reverse the nodes in groups of k in a given linked list, where k is a positive integer, and at most the length of the linked list. If any remaining nodes are not part of a group of k, they should remain in their original order.

It is not allowed to change the values of the nodes in the linked list. Only the order of the nodes can be modified.

Note: Use only O(1) extra memory space.

Constraints

Let n be the number of nodes in a linked list.

• 1 ≤ k ≤ n ≤ 500
• 0 ≤ Node.value ≤ 1000

Examples

Example 1

Input: head = [1, 2, 3, 4, 5, 6, 7, 8] k = 3

Explanation:

• First group of 3: [1, 2, 3] → reversed to [3, 2, 1]
• Second group of 3: [4, 5, 6] → reversed to [6, 5, 4]
• Remaining nodes [7, 8] are fewer than k, so they stay in original order.

Output: [3, 2, 1, 6, 5, 4, 7, 8]

Example 2

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

Explanation:

• First group of 2: [1, 2] → reversed to [2, 1]
• Second group of 2: [3, 4] → reversed to [4, 3]
• Remaining node [5] is fewer than k, so it stays as is.

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