leetcode--010 Linked List Cycle II

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