/**

 * 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 fast = head, slow = head;

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

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

            fast = fast.next.next;

            slow = slow.next;

            if(fast == slow){

                return true;

            }

        }

        return false;

    }

}