spfa判断负环模板

bool spfa()
{
     
    memset(dist, 0, sizeof dist);
    memset(cnt, 0, sizeof cnt);
    memset(st, 0, sizeof st);

    int hh = 0, tt = 0;
    for (int i = 1; i <= n; i ++ )
    {
     
        q[tt ++ ] = i;
        st[i] = true;
    }

    while (hh != tt)
    {
     
        int t = q[hh ++ ];
        if (hh == N) hh = 0;
        st[t] = false;

        for (int i = h[t]; ~i; i = ne[i])
        {
     
            int j = e[i];
            if (dist[j] > dist[t] + w[i])
            {
     
                dist[j] = dist[t] + w[i];
                cnt[j] = cnt[t] + 1;
                if (cnt[j] >= n) return true;
                if (!st[j])
                {
     
                    q[tt ++ ] = j;
                    if (tt == N) tt = 0;
                    st[j] = true;
                }
            }
        }
    }

    return false;
}