Prim
AcWing 858. Prim算法求最小生成树

#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];
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[t] > dist[j]))
                t = j;

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

        for (int j = 1; j <= n; j++) dist[j] = min(dist[j], g[t][j]);
        //if (i) res += dist[t];
        //res不能在这个位置，对于坑点数据 4 4 -10
        //4这个点存在负环，当t==j的时候，min(dist[t], g[t][t])会把自己更新小了
        st[t] = true;  //下标容易打错
    }
    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);  //无向图是一种特殊的有向图
    }

    int t = prim();

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

    return 0;
}