Reverse Nodes in k-Group [LeetCode]

Problem Description: http://oj.leetcode.com/problems/reverse-nodes-in-k-group/

Basic Idea: Do it like reverse a linked list with a counter started by k. Record the tail at the first, then let the tail-> next be the head of rest linked list. This is the recursive version, which is much easier than interative version.

 /**
  * 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(k <= )
             return head;

         ListNode *iterate = head;
         int num = ;
         while(iterate != NULL) {
             num ++;
             iterate = iterate -> next;
         }
         if(num < k)
             return head;

         ListNode * new_head = NULL;
         ListNode * tail = head;
         int count = k;
         while(head != NULL && count > ) {
             ListNode *pre_head = new_head;
             new_head = head;
             head = head->next;
             count --;
             new_head->next = pre_head;
         }

         if(head != NULL)
             tail->next = reverseKGroup(head, k);

         return new_head;
     }
 };