kruskal算法（附注释）：将边按照权值由小到大排列，然选取一条边，如果和前面的边一起不能构成环则加入该边，否则舍弃改变，执行下一条边，这里的环就是由集合来表示的，nodes[i]=j 表示i节点的顶点时j，-kruskal algorithm (annotated): the edge of the right in accordance with the value of small to large order, ran select an edge, if and front edges are added together do not constitute part of the edge, otherwise discard changes, the next one edge, where the ring is expressed by the collection of, nodes [i] = j　that when the vertex i node j,