141. Linked List Cycle

 1 public class Solution {
 2     public boolean hasCycle(ListNode head) {
 3         if (head == null || head.next == null) return false;
 4         ListNode slow = head;
 5         ListNode fast = head.next;
 6 
 7         while(fast != null && fast.next != null) {
 8             if (slow == fast) return true;
 9             slow = slow.next;
10             fast = fast.next.next;
11         }
12         return false;
13     }
14 }