Linked List Cycle II

fastrunner and slowrunner trick
 1 public class Solution {
 2     public ListNode detectCycle(ListNode head) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         ListNode fastrunner = head;
 6         ListNode slowrunner = head;
 7         while(fastrunner != null && fastrunner.next != null)
 8         {
 9             fastrunner = fastrunner.next.next;
10             slowrunner = slowrunner.next;
11             if(slowrunner == fastrunner)
12                 break;
13         }
14         if(fastrunner == null || fastrunner.next == null)
15             return null;
16         slowrunner = head;
17         while(slowrunner != fastrunner)
18         {
19             slowrunner = slowrunner.next;
20             fastrunner = fastrunner.next;
21         }
22         return slowrunner;
23     }
24 }