leetcode-006 detect cycle

 package leetcode;

 public class DetectCycle {
     public ListNode detectCycle(ListNode head) {
         ListNode s=head;
         ListNode f=head;
         while(f!=null&&f.next!=null){
             s=s.next;
             f=f.next.next;
             if(s==f){
                 break;
             }
         }
         if(f==null||f.next==null)
             return null;
         s=head;
         while(s!=f){
             s=s.next;
             f=f.next;
         }
         return s;
     }
 }