/**
 * https://leetcode-cn.com/problems/reverse-nodes-in-k-group/
 * 
 * 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) {
        
        // 1. k1
        if(head==null || head.next==null || k