环形链表

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