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 }