80分求助

#include<bits/stdc++.h>
using namespace std;

int a,b,w,n,m,t,dist[2001]={},cnt[2001]={},flag=0;
bool visited[2001]={};
vector<int> son[2001],weight[2001];
list<int> l;

int main(){
  cin>>t;
  for(int k=1;k<=t;k++){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++){
      dist[i]=1e9;
    }
    memset(cnt,0,sizeof(cnt));
    memset(visited,0,sizeof(visited));
    for(int i=1;i<=n;i++){
      son[i].clear();
      weight[i].clear();
    }
    for(int i=1;i<=m;i++){
      scanf("%d%d%d",&a,&b,&w);
      if(w<0){
        son[a].push_back(b);
        weight[a].push_back(w);
      }
      else{
        son[a].push_back(b);
        son[b].push_back(a);
        weight[a].push_back(w);
        weight[b].push_back(w);
      }
    }
    l.push_back(1);
    dist[1]=0;
    flag=0;
    while(!l.empty() && flag==0){
      int u=l.front();
      for(int i=0;i<son[u].size();i++){
        if(dist[son[u][i]]>dist[u]+weight[u][i]){
          dist[son[u][i]]=dist[u]+weight[u][i];
          if(!visited[son[u][i]]){
            l.push_back(son[u][i]);
            visited[son[u][i]]=1;
            cnt[son[u][i]]++;
            if(cnt[son[u][i]]==n){
              cout<<"YE5"<<endl;
              flag=1;
              break;
            }
          }
        }
      }
      l.pop_front();
      visited[u]=0;
    }
    if(flag==0){
      cout<<"N0"<<endl;
    }
  }
  return 0;
}