说明：  在一个具有几个顶点的连通图G中，如果存在子图G 包含G中所有顶点和一部分边，且不形成回路，则称G 为图G的生成树，代价最小生成树则称为最小生成树
(If there subgraph G ' contains all vertices in G, and part of the side, and does not form a loop, then G' in a few vertices connected graph G is the spanning tree of a graph G consideration of the minimum spanning tree is called minimum spanning tree)