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