说明：  弗利姆算法，普利姆(Prim)算法求最小生成树，也就是在包含n个顶点的连通图中，找出只有(n-1)条边包含所有n个顶点的连通子图，也就是所谓的极小连通子图
(Prim algorithm to find the minimum spanning tree, that is, in the connected graph containing N vertices, only (n-1) edge containing all n vertices of the connected subgraph, which is the so-called minimal connected subgraph)