给定n个城市之间的有向图，每条边是以字母标示的，输出在每条可行线路u->v中，其所经过的每一段线路的相同的字母。经典的传递闭包问题。-There are n cities between a given directed graph, each edge is marked letters in each line feasible u->　v, and they go through the same section of the line output of each letter. Classic transitive closure problem.