25. K 个一组翻转链表

leetcodehttps://leetcode.cn/problems/reverse-nodes-in-k-group/

解题思路

递归到最后面一段，从最后面开始翻转，每次返回翻转后的头节点。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode now=null;
        now=head;
        int cnt=0;
        while(now!=null && cnt<k-1){//如果不到k，直接返回
            now=now.next;
            cnt+=1;
        }
        if(now==null){
            return head;
        }
        ListNode pre=reverseKGroup(now.next,k),next=now.next;//从最后面开始翻转
        while(cnt>=0){
            ListNode temp=head.next;
            head.next=pre;
            pre=head;
            head=temp;
            cnt-=1;
        }
        return now;//返回每段翻转后的头节点
    }
}