/**

  * Definition for singly-linked list.

  * struct ListNode {

  *     int val;

  *     ListNode *next;

  *     ListNode(int x) : val(x), next(NULL) {}

  * };

  */

 class Solution {

 public:

     bool hasCycle(ListNode *head) {

         if(head==NULL)

             return false;

         ListNode *slow=head,*fast=head;

         while(fast!=NULL&&fast->next!=NULL){

             fast=fast->next->next;

             slow=slow->next;

             if(fast==slow)

                 return true;

         }

         return false;

     }

 };