使用克鲁斯卡尔算法生成最小生成树，将输入的边按照权值大小进行排列，每条边的端点属于不同的集合，当选取的一条边的两个端点同属于一个集合时，此边放弃。不属于同一个集合则合并。

-Use kruskal algorithm to generate the minimum spanning tree, the edge of the input according to the weight, size of each edge endpoints belong to different collections, elected to take an edge of two endpoints with belongs to a collection, this side to give up. Do not belong to the same collection is merged.