class Solution {

 public:

     bool hasCycle(ListNode *head) {

         if (head == NULL) return false;

         ListNode *slow = head;

         if (head->next == NULL) return false;

         ListNode *fast = head->next;

         while (slow != fast)

         {

             if (slow != NULL)

                 slow = slow->next;

             if (fast != NULL)

                 fast = fast->next;

             if (fast != NULL)

                 fast = fast->next;

         }

         return slow != NULL;

     }

 };