记忆化搜索

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5000;
const int MOD = 80112002;
int n, m;
int inD[MAXN + 5]; // 记录每个点的入度
vector<int> e[MAXN + 5];
// f(u) 从 u 有多少条走到底的路径
int book[MAXN + 5];
int f(int u)
{
    if (book[u] != 0)
        return book[u];
    int res = 0;
    for (int v : e[u])
        res = (res + f(v)) % MOD;
    if (res == 0)
        res = 1;
    return book[u] = res;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        int u, v;
        cin >> u >> v;
        e[u].push_back(v);
        inD[v]++;
    }
    for (int i = 1; i <= n; i++)
        if (inD[i] == 0)
        {
            e[n + 1].push_back(i);
            inD[i]++;
        }
    cout << f(n + 1);
    return 0;
}