实现一个浏览器的功能。当输入BACK，输出刚访问过的网页，取出作为当前网址；若以前没有，则输出Ignored；还可输入FORWARD、VISIT等，实现相应的功能。-The realization of a browser function. When the input BACK, output has just visited the web page, retrieve the current URL　if not previously, then the output Ignored　also enter FORWARD, VISIT, etc., to achieve the corresponding functions.