P3385 【模板】负环（SPFA）

题目链接:https://www.luogu.com.cn/problem/P3385

题解

#include <bits/stdc++.h>
using namespace std;
const int N=1e5+10;
typedef long long ll;
int t,n;
int m;

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin>>t;
    while(t--)
    {
        cin>>n>>m;
        vector<vector<pair<int,int>>>graph(n+1);
        vector<int>dis(n+1,INT_MAX);
        vector<bool>enter(n+1,false);
        queue<int>que;
        vector<int>update(n+1,0);
        for(int i=0;i<m;i++)
        {
            int u,v,w;
            cin>>u>>v>>w;
            if(w>=0)
            {
                graph[u].push_back({v,w});
                graph[v].push_back({u,w});
            }
            else
            {
                graph[u].push_back({v,w});
            }
        }
        dis[1]=0;
        que.push(1);
        enter[1]=true;
        update[1]=1;
        int cmd=0;
        while(!que.empty())
        {
            int x = que.front();

            enter[x]=false;
            que.pop();
            for(auto edge:graph[x])
            {
                int to = edge.first;
                int ww = edge.second;
                if(dis[to]>dis[x]+ww)
                {
                        
                    dis[to]=dis[x]+ww;
                    if(!enter[to])
                    {
                        update[to]++;
                        que.push(to);
                        enter[to]=true;                       
                    }
                }  
                if(update[to]>=n)
                {
                    cmd=1;
                    break;
                }
            }
            if(cmd==1)break;
        }
        if(cmd==1)cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}