141. Linked List Cycle

Question:

Given a linked list, determine if it has a cycle in it.

idea：

1. Use two pointers, walker and runner.
2. walker moves step by step. runner moves two steps at time.
3. if the Linked List has a cycle walker and runner will meet at some
   point.

   public boolean hasCycle(ListNode head) {
     if(head==null) return false;
     ListNode walker = head;
     ListNode runner = head;
     while(runner.next!=null && runner.next.next!=null){
       walker = walker.next;
       runner = runner.next.next;
       if(walker==runner){
         return true;
       }
     }
     return false;
   }