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

Follow up:
Can you solve it without using extra space?

 //兩個速度指標
    public boolean hasCycle(ListNode head) {

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

           walker = walker.next;
           runner = runner.next.next;
           if(walker == runner){
               return true;
           }
       }
       return false;
    }