LeetCode-Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

 1 /**
 2  * Definition for singly-linked list.
 3  * class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public boolean hasCycle(ListNode head) {
14         if(head == null || head.next == null || head.next.next == null){
15             return false;
16         }
17         ListNode fast = head.next;
18         ListNode slow = head;
19         while(fast !=null && fast.next != null){
20             if(fast == slow){
21             return true;
22         }
23             fast = fast.next.next;
24             slow = slow.next;
25         }
26 
27         return false;
28         
29         
30     }
31 }