/**

  * Definition for singly-linked list.

  * struct ListNode {

  *     int val;

  *     ListNode *next;

  *     ListNode(int x) : val(x), next(NULL) {}

  * };

  */

 class Solution {

 public:

     ListNode* reverseKGroup(ListNode* head, int k) {

         if(head == NULL || k == ) return head;

         int len = getLen(head);

         ListNode *start = head, *lastTail = new ListNode(), *newhead = lastTail;

         newhead->next = head;

         for(int i = ; i < len / k; ++i) {

             lastTail->next = reverseGroup(start, k); // 和上一段尾巴接上

             lastTail = start;

             start = lastTail->next;

         }

         return newhead->next;

     }

     // 返回反转链表的头结点,顺便把尾部接到下一段头部

     ListNode* reverseGroup(ListNode* head, int k) {

         ListNode *p1 = head, *p2 = head->next;

         for(int i = ; i < k - ; ++i) {

             head->next = p2->next;

             p2->next = p1;

             p1 = p2;

             p2 = head->next;

         }

         return p1;

     }

     int getLen(ListNode *head, ListNode *end = NULL) {

         int ret = ;

         while(head != end) {

             ++ret;

             head = head->next;

         }

         return ret;

     }

 };