说明：  设计模式一（单子、工厂、策略模式）练习 1． 假设现在要设计一个贩卖各类书籍的电子商务网站的购物车系统。对所有的教材类图书 实行每本一元的折扣；对连环画类图书提供每本7%的促销折扣；而对非教材类的计算机图书有3%的折扣；对其余书没有折扣。使用策略模式、工厂模式、单子模式进行设计
(Design Mode 1 (List, Factory, Strategy Mode) Exercise 1. Suppose we are going to design a shopping cart system for e-commerce websites that sell all kinds of books. All textbook books are discounted at one yuan per book; comic books are discounted at 7% per book; computer books are discounted at 3% for non-textbook books; and other books are discounted at no rate. Design using strategy mode, factory mode and single mode)