141. Linked List Cycle
双指针
1 public boolean hasCycle(ListNode head) { 2 if(head == null) { 3 return false; 4 } 5 ListNode runner = head; 6 ListNode walker = head; 7 while(runner.next != null && runner.next.next != null) { 8 walker = walker.next; 9 runner = runner.next.next; 10 if(walker == runner) { 11 return true; 12 } 13 } 14 return false; 15 }