Problem Linked List Cycle II

 1     public ListNode detectCycle(ListNode head) {
 2         if (head == null) return null;
 3         if (head.next == null || head.next.next == null) return null; 
 4         ListNode tortoise = head.next;
 5         ListNode hare = head.next.next;
 6         while (tortoise != hare) {
 7             if (tortoise.next == null) return null;
 8             tortoise = tortoise.next;
 9             if (hare.next == null || hare.next.next == null) return null;
10             hare = hare.next.next;
11         }
12 
13         tortoise = head;
14         while(tortoise != hare) {
15             tortoise = tortoise.next;
16             hare = hare.next;
17         }
18 
19         return tortoise;
20     }