Reverse Nodes in k-Group

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 they are.

You may not alter the values in the list's nodes, only nodes themselves may be changed.

Examples

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]

Example 3:

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

Constraints

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

Expected Complexity

• Time: O(n)
• Space: O(1) for iterative, O(n/k) for recursive