P4017 最大食物链计数(记忆化搜索)

#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <cstring>
#include <set>
#include <cmath>
#include <map>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
const int MN = 65005;
const int MAXN = 1000005;
const int INF = 0x3f3f3f3f;
#define IOS ios::sync_with_stdio(false);


const int mod = 80112002;
int n, m;
ll ans;
vector<int> eat[5005];
int eater[5005];
int dp[5005];
int ate[5005];

int dfs(int x) {
	int num = 0;
	if (!ate[x]) {
		return 1;
	}
	if (dp[x]) {
		return dp[x];
	}
	for (auto e : eat[x]) {
		(num += dfs(e)) %= mod;
	}
	return dp[x] = num;
}

int main() {
	int t1, t2;
	scanf("%d %d", &n, &m);
	for (int i = 1; i <= m; i++) {
		scanf("%d %d", &t1, &t2);
		eat[t1].push_back(t2);
		eater[t2] = 1;
		ate[t1] = 1;
	}
	for (int i = 1; i <= n; i++) {
		if (!eater[i]) {
			(ans += dfs(i)) %= mod;
		}
	}
	printf("%lld", ans);
	return 0;
}