spfa 判负环

#include<bits/stdc++.h>
using namespace std;
const int N = 2020, M = 1e4 + 10, INF = 0x3f3f3f3f;
int n, m, h[N], idx, dis[N], cnt[N];
bool vis[N];
struct edge{
    int t, l, n;
    edge(){}
    edge(int tt, int ll, int nn)
    {
        t = tt, l = ll, n = nn;
    }
}e[M];

void addEdge(int ff, int tt, int ll)
{
    e[idx] = edge(tt, ll, h[ff]);
    h[ff] = idx ++ ;
}

bool spfa()
{
    queue<int> q;
    for(int i = 1; i <= n; i ++ )
    {
        q.push(i);
        vis[i] = true;
    }
    while(!q.empty())
    {
        int f = q.front();
        q.pop();
        vis[f] = false;
        for(int i = h[f]; ~i; i = e[i].n)
        {
            int t = e[i].t, l = e[i].l;
            if(dis[t] > dis[f] + l)
            {
                dis[t] = dis[f] + l;
                cnt[t] = cnt[f] + 1;
                if(cnt[t] >= n) return true;
                if(!vis[t])
                {
                    vis[t] = true;
                    q.push(t);
                }
            }
        }
    }
    return false;
}
int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    for(int i = 0; i < m; i ++ )
    {
        int f, t, l;
        cin >> f >> t >> l;
        addEdge(f, t, l);
    }

    if(spfa()) puts("Yes");
    else puts("No");


    return 0;
}