/**

 * 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){

            return false;

        }

        ListNode slow=head;

        ListNode fast=head;

        while(fast!=null&&fast.next!=null){

            slow=slow.next;

            fast=fast.next.next;

            if(slow==fast){

                return true;

            }

        }

        return false;

    }

}