/**

 * 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) {

        ListNode* fast = head;

        ListNode* slow = head;

        while(fast && fast -> next){

            fast = fast -> next -> next;

            slow = slow -> next;

            if(fast == slow) return true;

        }

        return false;

    }

};