prim 板子

#include <bits/stdc++.h>
using namespace std;
const int N = 510;
const int inf = 0x3f3f3f3f;

int g[N][N];
int dist[N];
bool st[N];

int n, m;

int prim() {
    memset(dist, inf, sizeof dist);
    int ans = 0;
    // 首先选择1号作为根节点
    st[1] = true;
    for (int j = 1; j <= n; ++j) dist[j] = min(dist[j], g[1][j]);
    // 再选择n - 1个结点
    for (int i = 1; i < n; ++i) {
        int t = 0;
        for (int j = 1; j <= n; ++j) {
            if (!st[j] && dist[j] < dist[t]) {
                t = j; // 找到当前未加入集合的最小距离结点
            }
        }
        if (dist[t] == inf) return inf;
        ans += dist[t];
        st[t] = true;
        for (int j = 1; j <= n; ++j) {
            dist[j] = min(dist[j], g[t][j]); // 更新到集合的点
        }
    }
    return ans;
}


int main () {
    cin >> n >> m;
    int a, b, c;
    // 邻接矩阵的初始化
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= n; ++j) {
            if (i == j) g[i][j] = 0;
            else g[i][j] = inf;
        }
    }
    for (int i = 0; i < m; ++i) {
        cin >> a >> b >> c;
        g[a][b] = min(g[a][b], c);
        g[b][a] = min(g[b][a], c);
    }
    int ans = prim();
    if (ans == inf) cout << "impossible" << endl;
    else cout << ans << endl;
    return 0;
}