O(n+m)

int h[N], e[M], ne[M], idx; //memset(h, -1, sizeof h);
int din[N];                 //入度
int q[N];                   //res

void add(int a, int b)
{
    din[b]++;
    e[idx] = b;
    ne[idx] = h[a];
    h[a] = idx++;
}

bool topsort() //检查是否为有向无环图
{
    int hh = 0, tt = -1;
    for (int i = 1; i <= n; i++) //起点
    {
        if (!din[i])
        {
            q[++tt] = i;
        }
    }
    while (hh <= tt)
    {
        int t = q[hh++];
        for (int i = h[t]; ~i; i = ne[i])
        {
            int j = e[i];
            din[j]--;
            if (!din[j])
            {
                q[++tt] = j;
            }
        }
    }
    return tt == n - 1;
}