Problem Statement

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

Here, 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, the remaining nodes at the end should stay as they are.

Note: You can only change the nodes themselves, not the values inside them.

Examples

1. Example 1:

   • Input: head = [1, 2, 3, 4, 5, 6], k = 3
   • Expected Output: [3, 2, 1, 6, 5, 4]
   • Justification: The list is reversed in groups of three. First group [1, 2, 3] becomes [3, 2, 1] and the second group [4, 5, 6] becomes [6, 5, 4].

2. Example 2:

   • Input: head = [1, 2, 3, 4, 5], k = 2
   • Expected Output: [2, 1, 4, 3, 5]
   • Justification: The list is reversed in groups of two. First group [1, 2] becomes [2, 1] and the second group [3, 4] becomes [4, 3]. The last node [5] remains unchanged.

3. Example 3:

   • Input: head = [1, 2, 3, 4, 5, 6, 7], k = 4
   • Expected Output: [4, 3, 2, 1, 5, 6, 7]
   • Justification: The list is reversed in groups of four. First group [1, 2, 3, 4] becomes [4, 3, 2, 1]. The remaining nodes [5, 6, 7] are left as they are because their count is less than k.

Constraints:

• The number of nodes in the list is n.
• 1 <= k <= n <= 5000
• 0 <= Node.val <= 1000

Try it yourself

Try solving this question here: