Leetcode || Reverse Nodes in k-Group

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

先计算长度，再分组，每组中node翻转，翻转用交换收尾方法

package pack;

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        int len = lenListNode(