#include<bits/stdc++.h>
#pragma GCC optimize(3)
#define maxn 10000
#define maxm 10000
#define inf 2147483647
using namespace std;
int m, n;
int num = 0;
struct eg
{
    int to;
    int cost;
    int next;
};
eg side[maxm];
int head[maxn], cnt[maxn], dis[maxn];
bool vis[maxn], flag = 0;
void addeg(int f, int t, int cost)
{
    side[++num].next = head[f];
    side[num].to = t;
    side[num].cost = cost;
    head[f] = num;
}
void spfa(int g)
{

    dis[g] = 0;
    vis[g] = 1;
    queue<int>q;
    q.push(g);
    int x;
    while(!q.empty())
    {
        x = q.front();
        q.pop();
        vis[x] = 0;
        for(int i = head[x]; i != 0; i = side[i].next)
        {
            if(dis[x] + side[i].cost < dis[side[i].to])
            {
                dis[side[i].to] = dis[x] + side[i].cost;
                cnt[side[i].to] = cnt[x] + 1;
                if(cnt[side[i].to] >= n)
                {
                    flag = 1;
                    return ;
                }
                if(!vis[side[i].to])
                {
                    q.push(side[i].to);
                    vis[side[i].to] = 1;
                }
            }
        }
    }
    return ;
}

int main()
{
    int T;
    int f, t, cost;
    scanf("%d", &T);
    while(T--)
    {
        num = 0;
        scanf("%d%d", &n, &m);
        // for(int i = 1; i <= m; i++)
        //     side[i].to = 0, side[i].next = 0, side[i].cost = 0;
        for(int i = 0; i <= n; i++)
            dis[i] = 0, cnt[i] = 0, vis[i] = 0, head[i] = 0, dis[i] = 0;
        for(int i = 1; i <= m; i++)
        {
            scanf("%d%d%d", &f, &t, &cost);
            if(cost < 0)
            {
                addeg(f, t, cost);
            }
            else
            {
                addeg(f, t, cost);
                addeg(t, f, cost);
            }
        }
        flag = 0;
        for(int i = 1; i <= n; i++)
        {
            spfa(i);
            if(flag)break;
        }
        if(flag) puts("YE5");
        else puts("N0");
    }
}
 

【模板】SPFA判負環