public void MaxDepth_1()
{
var root = new TreeNode(3);
root.left = new TreeNode(9);
root.right = new TreeNode(20);
root.right.left = new TreeNode(15);
root.right.right = new TreeNode(7);
var expected = 3;
var actual = _target.MaxDepth(root);
Assert.Equal(expected, actual);
}