leetcode刷题34

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