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);
        }
        public void Can_postorder_big_traverse()
        {
            // Arrange
            BinarySearchTree <int> testTree = new BinarySearchTree <int>();

            testTree.Add(5);
            testTree.Add(7);
            testTree.Add(9);
            testTree.Add(1);
            testTree.Add(12);
            testTree.Add(2);
            IEnumerable <int> expected = new int[] { 2, 1, 12, 9, 7, 5 };


            // Act
            IEnumerable <int> actual = testTree.PostOrder(testTree.ReturnRoot());

            // Assert
            Assert.Equal(expected, actual);
        }