//解题思路：
    //递归判断左右子节点哪个更大
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        int leftDepth = TreeDepth(root.left);
        int rightDepth = TreeDepth(root.right);
        int depth = leftDepth > rightDepth ? leftDepth : rightDepth;
        return depth + 1;
    }