public void Can_preorder_traverse()
{
// Arrange
BinarySearchTree <int> testTree = new BinarySearchTree <int>();
testTree.Add(5);
testTree.Add(9);
testTree.Add(1);
IEnumerable <int> expected = new int[] { 5, 1, 9 };
// Act
IEnumerable <int> actual = testTree.PreOrder(testTree.ReturnRoot());
// Assert
Assert.Equal(expected, actual);
}