Reverse Nodes in k-Group

The key idea

Reverse each block of k nodes in place, but only after you confirm a full group of k exists ahead. Keep a prev_group_tail that points at the last node of the already-finished part, so each reversed block can be stitched back onto the chain in the right order.

Problem

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 the nodes that are left out at the end should remain as is.

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

Constraints

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

Examples

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

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

Complexity

Time: O(n) Space: O(1)

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