bool spfa()
{
    int tot = 0;
    queue<int> q;
    for (int i = 1; i <= n; i ++ )
    {
        q.push(i);
        st[i] = true;
    while (q.size())
    {
        auto t = q.front();
        q.pop();
        st[t] = false;
        for (int i = h[t]; i != -1; 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(++tot > 2 * n)
                if (cnt[j] >= n) return true;      
                if (!st[j])
                {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }

    return false;
}