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;

     }

 }