prim算法求最小生成树 稀疏图

https://www.acwing.com/problem/content/860/

#include<iostream>
#include<string.h>
using namespace std;
const int N=510;
int g[N][N],dis[N];
int n,m;
bool st[N];


int prim(){
    int res=0;
    memset(dis,0x3f,sizeof dis);
    for(int i=0;i<n;i++){
        int t=-1;
        for(int j=1;j<=n;j++)
            if(!st[j]&&(t==-1||dis[t]>dis[j]))
                t=j;
        if(i&&dis[t]==0x3f3f3f3f) return 0x3f3f3f3f;
        if(i) res+=dis[t];
        st[t]=true;
        for(int j=1;j<=n;j++)
            dis[j]=min(dis[j],g[t][j]);
    }
    return res;
}
int main(){
    cin>>n>>m;
    memset(g,0x3f,sizeof g);
    for(int i=0;i<m;i++){
        int a,b,c;
        cin>>a>>b>>c;
        g[a][b]=g[b][a]=min(g[a][b],c);
    }
    int t=prim();
    if(t>0x3f3f3f3f/2) cout<<"impossible"<<endl;
    else cout<<t<<endl;
    return 0;
}