public void getTotalAmountTest_第一集買了一本_第二集也買了一本_價格應為190()
        {
            var target = new PotterShoppingCart();

            target.addToCart(1);
            target.addToCart(2);
            var actual   = target.getTotalAmount();
            var expected = 190;

            Assert.AreEqual(expected, actual);
        }
        public void getTotalAmountTest_一二三集各買了一本_價格應為270()
        {
            var target = new PotterShoppingCart();

            target.addToCart(1);
            target.addToCart(2);
            target.addToCart(3);
            var actual   = target.getTotalAmount();
            var expected = 270;

            Assert.AreEqual(expected, actual);
        }
        public void getTotalAmountTest_一次買了整套_一二三四五集各買了一本_價格應為375()
        {
            var target = new PotterShoppingCart();

            target.addToCart(1);
            target.addToCart(2);
            target.addToCart(3);
            target.addToCart(4);
            target.addToCart(5);
            var actual   = target.getTotalAmount();
            var expected = 375;

            Assert.AreEqual(expected, actual);
        }