public void BinaryTreeTest_PostOrder()
        {
            BinaryTree<int> tree = new BinaryTree<int>();
            String order = String.Empty;

            tree.Add(6);
            tree.Add(3);
            tree.Add(9);
            tree.Add(1);
            tree.Add(4);

            foreach (var item in tree.Order(TreeOrder.Postorder))
            {
                order += item;
            }

            Assert.AreEqual("96431", order);
        }