题解 | #牛群的重新分组#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param k int整型 
     * @return ListNode类
     */
    public ListNode reverseKGroup (ListNode head, int k) {
        // write code here
        if(k == 0 || head == null) return head;
        ListNode dummyNode = new ListNode(-1);
        dummyNode.next = head;
        ListNode pre = dummyNode;
        ListNode tail = dummyNode;
        while(tail != null){
            for(int i = 0 ; i < k && tail != null; i ++){
                tail = tail.next;
            }
            if(tail == null) break;
            ListNode next = tail.next;
            tail.next = null;
            ListNode start = pre.next;
            pre.next = reverse(start);
            pre = start;
            tail = start;
            start.next = next;
        }
        return dummyNode.next;
    }

    public ListNode reverse(ListNode head){
        if(head == null || head.next == null){
            return head;
        }
        ListNode node = reverse(head.next);
        head.next.next = head;
        head.next = null;
        return node;
    }
}

兄弟们莽就完事了