141. Linked List Cycle

 1     public boolean hasCycle(ListNode head) {
 2         if (head == null) return false;
 3         ListNode walker = head;
 4         ListNode runner = head;
 5         while (runner.next !=null && runner.next.next != null)
 6         {
 7             walker = walker.next;
 8             runner = runner.next.next;
 9             if (walker == runner) return true;
10         }
11         return false;        
12     }