运用graph；寻找最短路径（最短时间/最少换乘）(Suppose following weighted directed graph representing a local subway route network, the vertices in the graph represent the important sites in the area, the arcs represents the existing subway routs, and the weight represent the time duration from one site to another one. Try to design a subway route query system to help the users find
(a) the way to go from one site to another one in shortest time(weighted shortest path)
(b) the way to go from one site to another one with minimal transfer(unweighted shortest path), for the routes with same transfer times choose the one costing shortest time.)