25. Reverse Nodes in k-Group

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LeetCode

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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 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.

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        int count = k;

        ListNode end = head;
        // 移动到末尾
        while(count != 0 && end != null){
            end = end.next;
            count--;
        }
        
        if(count == 0){
            end = reverseKGroup(end, k);
            for(int i = 0; i < k; i++){
                ListNode next = head.next;
                head.next = end;
                end = head;
                head = next;
            }
            head = end;
        }

        return head;
    }
}