1 public class Solution {
 2     public ListNode detectCycle(ListNode head) {
 3         if (head == null) {
 4             return head;
 5         }
 6         ListNode slow = head, fast = head;
 7         while (fast != null && fast.next != null) {
 8             slow = slow.next;
 9             fast = fast.next;
10             fast = fast.next;
11             if (slow == fast) {
12                 break;
13             }
14         }
15         if (fast == null || fast.next == null) {
16             return null;
17         }
18         fast = head;
19         while (fast != slow) {
20             fast = fast.next;
21             slow = slow.next;
22         }
23         return fast;
24     }
25 }