啊。你听我辩解~ 思路已经捋出来了，就是最后的计算结果出不来！！！玄关求助！！！急~

#include<bits/stdc++.h>
using namespace std;
//Cow Traffic
const int N = 50010;
vector <int> g[N];
int a[N], b[N], n, m, ans = -1e9, x, y;
bool f[N];
int main(){
	cin >> n >> m;
	for(int i = 1;i <= m;i++){
		cin >> x >> y;
		g[x].push_back(y);
		f[y] = true;
	}
	for(int v = 1;v <= m;v++)
		if(f[v]) g[0].push_back(v);
	a[0] = 1;
	for(int u = 0;u <= n;u++)
		for(int v : g[u]) a[v] += a[u];
	b[n] = 1;
	for(int u = n;u >= 0;u--)
		for(int v : g[u]) b[u] += b[v];
	for(int i = 1;i <= m;i++) ans = max(ans, a[i] * b[i+1]);
	cout << ans << endl;
	return 0;
}

拜托各位同（DA）学（LAO）们帮忙调调，谢谢啦~