/**
* 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) {
HashSet<ListNode> hashSet = new HashSet<ListNode>();
ListNode p;
p=head;
while(p!=null){
if(!hashSet.add(p)) return true;
p=p.next;
}
return false;
}
}