leetcode-006 detect cycle

 1 package leetcode;
 2 
 3 public class DetectCycle {
 4     public ListNode detectCycle(ListNode head) {
 5         ListNode s=head;
 6         ListNode f=head;
 7         while(f!=null&&f.next!=null){
 8             s=s.next;
 9             f=f.next.next;
10             if(s==f){
11                 break;
12             }
13         }
14         if(f==null||f.next==null)
15             return null;
16         s=head;
17         while(s!=f){
18             s=s.next;
19             f=f.next;
20         }
21         return s;
22     }
23 }