/**
     * 返回中序遍历结果
     *
     * @param root
     * @return
     */
     //递归解法
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        //处理边界情况
        if (root == null) {
            return result;
        }
        List<Integer> leftList = inorderTraversal(root.left);
        result.addAll(leftList);
        result.add(root.val);
        List<Integer> rightList = inorderTraversal(root.right);
        result.addAll(rightList);
        return result;
    }