kruskal算法相比prim算法思路简单，不用处理边界问题，不用堆优化，所以一般稀疏图都用Kruskal。

Kruskal算法时间复杂度O(mlogm)

每条边存结构体里，排序需要在结构体里重载小于号

判断a，b点是否连通以及将点假如集合中需要并查集的知识

#include<iostream>
#include<cstring>
#include<algorithm>using namespace std;const int N = 100010, M = 200010;int n, m;
int p[N];struct Edge
{int a, b, w;bool operator< (const Edge& W)const{return w < W.w;}
}edges[M];int find(int x)
{if(p[x] != x) p[x] = find(p[x]);return p[x];
}int Kruskal()
{int res = 0,cnt = 0;for(int i = 1; i <= n; i ++ ){p[i] = i;}for(int i = 0; i < m; i ++ ){int a = find(edges[i].a), b = find(edges[i].b);if(a != b){p[a] = b;res += edges[i].w;cnt ++ ;}}if(cnt < n - 1) return 0;else return res;
}int main()
{cin >> n >> m;for(int i = 0; i < m; i ++ ){int a, b, w;cin >> a >> b >> w;edges[i].a = a, edges[i].b = b, edges[i].w = w;}sort(edges, edges + m);int t = Kruskal();if(!t) cout << "impossible" << endl;else cout << t << endl;return 0;
}