var hasCycle = function(head) {
        //1.判断不存在的情况
        if (!head || head.next) {
            return false
        }
        //2.双指针循环
        var fast = head.next
        var slow = head
        //2.1快指针每次走两步，慢指针每次走一步，如果快指针一直有值，那就证明是环形，直到两个相等就退出
        while(fast && fast.next){
            if(fast == slow) {
                return true
            }
            fast = fast.next.next
            slow = slow.next
        }
        //2.2否则，就不是环形
        return false
    };

var hasCycle = function(head) {
        while(head != null){
            if(head == head.next){
                return true;
            }
            if(head.next != null){
                head.next = head.next.next;
            }
            head = head.next;
        }
        return false;
    };