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 <= 1)
            return head;
            
        ListNode *iterate = head;
        int num = 0;
        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 > 0) {
            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;
    }
};