说明：  Kruskal算法实现 给定一个无向图，如果它任意两个顶点都联通并且是一棵树，那么我们就称之为生成树(Spanning Tree)。如果是带权值的无向图，那么权值之和最小的生成树，我们就称之为最小生成树(MST, Minimum Spanning Tree)。
(Kruskal algorithm implements a given undirected graph. If any two vertices are connected and a tree, then we call it a spanning tree. If it is a undirected graph with weights, then the spanning tree with the smallest sum of weights is called MST (minimum spanning tree).)