142. Linked List Cycle II

 1     public ListNode detectCycle(ListNode head) {
 2         if(head == null) {
 3             return null;
 4         }
 5         ListNode runner = head;
 6         ListNode walker = head;
 7         boolean hasCycle = false;
 8         while(runner.next != null && runner.next.next != null) {
 9             walker = walker.next;
10             runner = runner.next.next;
11             if(walker == runner) {
12                 hasCycle = true;
13                 break;
14             }
15         }
16         ListNode res = head;
17         if(hasCycle) {
18             while(walker != res) {
19                 res = res.next;
20                 walker = walker.next;
21             }
22             return res;
23         }
24         return null;
25     }