题解 | 链表中的节点每k个一组翻转

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 (head == null) {
            return null;
        }
        ListNode tail = head;
        int i = 0;
        while (tail !=null && i < k) {
            tail = tail.next;
            i++;
        }
        if(i < k){
            return head;
        }
        ListNode pre = null;
        ListNode temp = null;
        ListNode cur = head;
        int count = 0;
        while (count < k) {
            temp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = temp;
            count++;
        }
        head.next = reverseKGroup(tail, k);
        return pre;
    }
}