洛谷P3366 【模板】最小生成树 RE三个点为什么

Question

    // prim 算法求最小生成树 
    #include <cstdio>
    #include <string>
    #include <cstring>
    #include <iostream>
    #include <algorithm>
    #define INF 0x3f3f3f3f
    using namespace std;
    const int maxn = 505;
    int a[maxn][maxn];
    int vis[maxn],dist[maxn];
    int n,m,u,v,w;
    long long sum = 0;
    int prim(int pos) {
        dist[pos] = 0;
        for(int i = 1; i <= n; i ++) {
            int cur = -1;
            for(int j = 1; j <= n; j ++)
                if(!vis[j] && (cur == -1 || dist[j] < dist[cur])) cur = j;
            if(dist[cur] >= INF) return INF;
            sum += dist[cur];
            vis[cur] = 1;
            for(int k = 1; k <= n; k ++) 
                if(!vis[k]) dist[k] = min(dist[k],a[cur][k]);
        }
        return sum;
    }
    int main(void) {
        scanf("%d%d",&n,&m);
        memset(a,0x3f,sizeof(a));                                                                 
        memset(dist,0x3f,sizeof(dist));
        for(int i = 1; i <= m; i ++) {
            scanf("%d%d%d",&u,&v,&w);
            a[u][v] = min(a[u][v],w);
            a[v][u] = min(a[v][u],w);
        }
        int value = prim(1);
        if(value >= INF) puts("orz");
        else printf("%lld\n",sum);
        return 0;
    }