prim求助

#include <cstdio>
#include <iostream>
#include <cmath>
using namespace std;
#define N 2000005
int n,m,cnt=0,sum=0;
int dis[N],vis[N];
int u[N],v[N],w[N],first[N],nxt[N];
int p[N];
int main(){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++){
        first[i]=-1;
        dis[i]=1e9;
        vis[i]=0;
    }
    for(int i=1;i<=m*2;i=i+2){
        scanf("%d%d%d",&u[i],&v[i],&w[i]);
        v[i+1]=u[i];
        u[i+1]=v[i];
        w[i+1]=w[i];
        nxt[i]=first[u[i]];
        first[u[i]]=i;
        nxt[i+1]=first[u[i+1]];
        first[u[i+1]]=i+1;
    }
    vis[1]=1;
    int k=first[1];
    while(k!=-1){
        dis[u[k]]=w[k];
        k=nxt[k];
    }
    int sum=0;
    while(cnt<n){
        int minn=N;
        int j;
        for(int i=1;i<=n;i++){
            if(dis[i]<minn&&vis[i]==0){
                minn=dis[i];
                j=i;
            }
        }
        cnt++;
        sum+=minn;
        vis[j]=1;
        k=first[j];
        while(k!=-1){
            dis[u[k]]=min(dis[u[k]],w[k]);
            k=nxt[k];
        }
    }
    cout<<sum;
    return 0;
}