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。?后退（BACK）：如果backward栈为空，则该命令被忽略。否则，将当前页面压入forward栈，并从backward栈中弹出一个页面作为当前页面。
?前进（FORWARD）：如果forward栈为空，则该命令被忽略。否则，将当前页面压入backward栈，并从forward栈中弹出一个页面作为当前页面。
?访问某网址（VISIT <URL>）：将当前页面压入backward栈，并将此次访问的网页作为当前页面，清空forward栈。(It is a common function of Web browser to carry forward and backward in web pages recently accessed by users. One way to achieve this function is to use two stack (backward stack and forward stack) to store user access URLs.Back up backup: if the backward stack is empty, the command is ignored. Otherwise, press the current page into the forward stack and pop up a page from the backward stack as the current page.

Forward forward: if the forward stack is empty, the command is ignored. Otherwise, press the current page into the backward stack and pop up a page from the forward stack as the current page.

Access to a web site (VISIT <URL>): press the current page into the backward stack and use the web page as the current page to clear the forward stack.)