与朴素版dijkstra算法思路类似，只不过dist数组存的是到集合的距离，而dijkstra存的是到源点的距离

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 510, INF = 0x3f3f3f3f;

int n, m;
int g[N][N];
int dist[N];//表示某点到集合的最小距离
bool st[N];


int prim()
{
    memset(dist,0x3f,sizeof dist);

    int res = 0;
    for(int i = 0;i<n;i++)
    {
        int t = -1;
        for(int j = 1;j<=n;j++)
        {
            if(!st[j] && (t == -1 || dist[j] < dist[t]))t = j;
        }

        if(i && dist[t] == INF)return INF;

        if(i)res += dist[t];
        st[t] = true;

        for(int j = 1;j<=n;j++)dist[j] = min(dist[j],g[t][j]);
    }
    return res;
}


int main()
{
    scanf("%d%d", &n, &m);

    memset(g, 0x3f, sizeof g);

    while (m -- )
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        g[a][b] = g[b][a] = min(g[a][b], c);
    }

    int t = prim();

    if (t == INF) puts("impossible");
    else printf("%d\n", t);

    return 0;
}