题目：若要在n个城市之间建立通信网络，只需要假设n－1条线路即可。如何以最低的经济代价建设这个通信网，是一个网的最小生成树问题。-topics : n if the city between communication networks, need only assume that n-1 line can be. How the lowest economic cost of building the communications network, the network is a minimum spanning tree problem.