Reverse Nodes in k-Group [LeetCode]

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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;

     }

 };