Reverse Nodes in k-Group

Problem Statement

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.

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]

Constraints

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

Code

Python3 Code

    class Solution:
    def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
        first = ListNode(0)
        first.next = head
        cur, fast = first, head
        while fast is not None:
            i = 0
            while i < k and fast is not None:
                fast = fast.next
                i += 1
            if i != k:
                break
            index = cur.next
            tail = fast
            while index is not fast:
                node = index
                index = index.next
                node.next = tail
                tail = node
            cur.next = tail
            while cur.next is not fast:
                cur = cur.next
        return first.next