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;

        ListNode *fast = head->next;

        while(fast)

        {

            if(slow == fast)

                return true;

            slow = slow->next;

            if(fast->next)

                fast = fast->next->next;

            else

                break;

        }

        return false;

    }

};