若无向图表示高速公路网，其中顶点表示城市，边表示城市之间的高速公路。试设计一个找路程序，获取两个城市之间的所有简单路径(If an undirected graph is used to express a highway network, the vertices represent the city, and the side represents the expressway between the cities. Try to design a path finding program to get all the simple paths between two cities)