Skip to main content

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.

Leetcode Link

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​

Python Code

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseKGroup(self, head, k):
        dummy = jump = ListNode(0)
        dummy.next = l = r = head

        while True:
            count = 0
            while r and count < k:   # use r to locate the range
                r = r.next
                count += 1
            if count == k:  # if size k satisfied, reverse the inner linked list
                pre, cur = r, l
                for _ in range(k):
                    cur.next, cur, pre = pre, cur.next, cur  # standard reversing
                jump.next, jump, l = pre, l, r  # connect two k-groups
            else:
                return dummy.next