/**

  * 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 || head->next == NULL) {

             return false;

         }

         ListNode * slow = head;

         ListNode * fast = head;

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

             slow = slow->next;

             fast = fast->next->next;

             if (fast == slow) {

                 return true;

             }

         }

         return false;

     }

 };