/**

  * Definition for singly-linked list.

  * class ListNode {

  *     int val;

  *     ListNode next;

  *     ListNode(int x) {

  *         val = x;

  *         next = null;

  *     }

  * }

  */

 public class Solution {

     public boolean hasCycle(ListNode head) {

         ListNode p1 = head;

         ListNode p2 = head;

         while(p2 != null){

             p1 = p1.next;

             p2 = p2.next;

             if(p2 != null)

                 p2 = p2.next;

             else

                 return false;

             if(p1 == p2)

                 return true;

         }

         return false;

     }

 }