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.

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *reverseKGroup(ListNode *head, int k) {
13         if(k <= 1)
14             return head;
15             
16         ListNode *iterate = head;
17         int num = 0;
18         while(iterate != NULL) {
19             num ++;
20             iterate = iterate -> next;
21         }
22         if(num < k)
23             return head;
24     
25         ListNode * new_head = NULL;
26         ListNode * tail = head;
27         int count = k;
28         while(head != NULL && count > 0) {
29             ListNode *pre_head = new_head;
30             new_head = head;
31             head = head->next;
32             count --;
33             new_head->next = pre_head;
34         }
35         
36         if(head != NULL)
37             tail->next = reverseKGroup(head, k);
38         
39         return new_head;
40     }
41 };