/**

  * 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) {

         ListNode *left = head, *right = head, *newHead = NULL;

         int len = ;

         while(right){

             len++;

             right = right->next;

         }

         if(len < k) return head;

         right = head;

         while(len >= k){

             ListNode *node = right;

             ListNode *h = right;

             right = right->next;

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

                 ListNode *next = right->next;

                 right->next = node;

                 node = right;

                 right = next;

             }

             if(newHead == NULL) newHead = node;

             else{

                 left->next = node;

                 left = h;

             }

             len -= k;

         }

         left->next = right;

         return newHead;

     }

 };