C

#include <stdio.h>
#include <string.h>

#define MAXSIZE 510
#define INF 0x3f3f3f3f
#define min(a,b) ((a)<(b)?(a):(b))
int n, m, visited[MAXSIZE], dist[MAXSIZE], graph[MAXSIZE][MAXSIZE];

void init() {
    scanf("%d%d", &n, &m);
    memset(dist, 0x3f, sizeof(dist));
    memset(graph, 0x3f, sizeof(graph));
    for (int i = 1; i <= n; ++i) {
        graph[i][i] = 0;
    }

    int u, v, w;
    while (m--) {
        scanf("%d%d%d", &u, &v, &w);
        graph[u][v] = min(graph[u][v], w);
        graph[v][u] = min(graph[v][u], w);
    }
}

int Prim() {
    dist[1] = 0;
    int ans = 0;
    for (int i = 1; i <= n; ++i) {
        int u = -1, curMin = INF;
        for (int j = 1; j <= n; ++j) {
            if (!visited[j] && dist[j] < curMin) {
                u = j;
                curMin = dist[j];
            }
        }

        if (u == -1) {
            return -1;
        }

        visited[u] = 1;
        ans += dist[u];
        for (int v = 1; v <= n; ++v) {
            if (!visited[v] && graph[u][v] != INF) {
                dist[v] = min(dist[v], graph[u][v]);
            }
        }
    }

    return ans;
}

int main() {
    init();
    int res = Prim();
    if (res == -1) {
        puts("impossible");
    } else {
        printf("%d\n", res);
    }

    return 0;
}

C++

#include <iostream>
#include <queue>
#include <vector>
using namespace std;

const int MAXSIZE = 510;
const int INF = 0x3f3f3f3f;
int n, m;
vector<bool> visited(MAXSIZE, false);
vector<int> dist(MAXSIZE, INF);
vector<vector<int> > graph(MAXSIZE, vector<int>(MAXSIZE, INF));
priority_queue<pair<int, int>, vector<pair<int, int> >, greater<pair<int, int> > > pq;

void init() {
    cin >> n >> m;
    for (int i = 1; i <= n; ++i) {
        graph[i][i] = 0;
    }

    int u, v, w;
    while (m--) {
        cin >> u >> v >> w;
        graph[u][v] = min(graph[u][v], w);
        graph[v][u] = min(graph[v][u], w);
    }
}

int Prim() {
    dist[1] = 0;
    int ans = 0, cnt = 0;

    pq.emplace(dist[1], 1);
    while (!pq.empty()) {
        auto [d, u] = pq.top();
        pq.pop();
        if (visited[u]) {
            continue;
        }

        visited[u] = true;
        ++cnt;
        ans += d;
        for (int v = 1; v <= n; ++v) {
            if (!visited[v] && graph[u][v] != INF && graph[u][v] < dist[v]) {
                dist[v] = graph[u][v];
                pq.emplace(dist[v], v);
            }
        }
    }

    return cnt == n ? ans : -1;
}

int main() {
    init();
    if (int res = Prim(); res == -1) {
        cout << "impossible" << endl;
    } else {
        cout << res << endl;
    }

    return 0;
}