在用户最近访问的网页中进行“前进”和“后退”是Web浏览器的常用功能，实现该功能的一种方式是使用两个栈（backward 栈和forward栈）来存储用户访问的网址("Forward" and "backward" is commonly used in Web browser users recently visited web pages in a way to achieve this function is to use two stacks (b