只判点1的负环
code:
#include <bits/stdc++.h>
using namespace std;
const int N = 2e3 + 10, M = 3e3 + 10;
int t;
int h[N], idx;
int cnt[N];
struct node
{
    int to, ne, w;
}e[M * 2];
int dist[N], st[N];

void add(int a, int b, int c){
    e[idx].to = b;
    e[idx].w = c;
    e[idx].ne = h[a];
    h[a] = idx++;
}

void spfa(int n){
    queue<int>q;
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    q.push(1);
    memset(st, 0, sizeof st);
    st[1] = 1;
    memset(cnt, 0, sizeof cnt);


    while(q.size()){
        auto t = q.front();
        q.pop();
        st[t] = false;

        for(int i = h[t];~i;i = e[i].ne){
            int j = e[i].to;
            if(dist[j] > dist[t] + e[i].w){
                dist[j] = dist[t] + e[i].w;
                cnt[j] = cnt[t] + 1;
                if(cnt[j] >= n){
                    cout << "YES" << endl;
                    return;
                }
                if(!st[j]){
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }
    cout << "NO" << endl;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin >> t;
    while(t--){
        idx = 0;
        memset(h, -1, sizeof h);
        int n, m;
        cin >> n >> m;
        for(int i = 1;i <= m;i++){
            int a, b, c;
            cin >> a >> b >> c;
            add(a, b, c);
            if(c >= 0)add(b, a, c);
        }
        spfa(n);
    }
    return 0;
}