[LeetCode]Binary Search Tree Iterator

可以把中序遍历的过程融入到next函数中，可以更快，下边的做法是先遍历完，比较垃圾

//记录节点值
    Queue<Integer> vals = new LinkedList<>();
    public BSTIterator(TreeNode root) {
        //构造函数不能主动调用所以不能递归，迭代中序遍历得到升序list
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty()||root!=null)
        {
            if (root!=null)
            {
                stack.push(root);
                root = root.left;
            }
            else
            {
                root = stack.pop();
                vals.offer(root.val);
                root = root.right;
            }
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !vals.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        return vals.poll();
    }