/**

* 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) {

if(head == null || head.next == null) return false;

ListNode p = head;

ListNode q = head.next.next;

while(q != null) {

if(p == q) return true;

p = p.next;

if(q.next == null) return false;

q = q.next.next;

}

return false;

}

}