图的遍历。若用有向网表示网页的链接网络，其中顶点表示某个网页，有向弧表示网页之间的链接关系。试设计一个网络蜘蛛系统，分别以广度优先和深度优先的策略抓取网页。-Graph traversal. If the web page to the network by indicating links network where vertices represent a page, there are links to the arcs represent the relationship between the pages. Try to design a web spider systems, respectively, breadth-first and depth-first strategy crawl the web.