拓扑排序计数 
记忆化搜索写法, 主要是统计各个点的入度和出度 然后遍历入读为0的点
和toposort思路一样
#include<bits/stdc++.h>
using namespace std;
const int N = 5010, M = 5e5 + 10, mod = 80112002;
int h[N], idx;
int n, m;
int idu[N], odu[N];
struct edge{
    int from, to, w;
    int ne;
}E[M];
int vis[N];

void add(int a, int b)
{
    E[idx].to = b;
    E[idx].ne = h[a];
    h[a] = idx++;
}

int dfs(int u)
{
    if(!odu[u]) return 1;
    if(vis[u])return vis[u];

    int ans = 0;
    for(int i = h[u];~i;i = E[i].ne)
    {
        int j = E[i].to;
        ans = (ans + dfs(j)) % mod;
    }
    vis[u] = ans;
    return vis[u];
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    memset(h, -1, sizeof h);

    cin >> n >> m;

    for(int i = 1;i <= m;i++)
    {
        int x, y;
        cin >> x >> y;
        idu[x]++, odu[y]++;
        add(y, x);
    }

    int res = 0;
//  cout << h[1] << endl;
//  cout << vis[1] << endl;
    for(int i = 1;i <= n;i++){
    if(!idu[i])res = (res + dfs(i)) % mod;
    }
    cout << res;
}