见代码注释

#include <bits/stdc++.h>
using namespace std;
const int N = 10005;
int n, m;
vector<int> vec[N];
int dfn[N], st[N], tp, scc[N], cnt, low[N], sc;
int sz[N];
bool inst[N];
int out[N];
void tarjan(int u) {
	dfn[u] = ++ cnt;
	low[u] = dfn[u];
	st[++ tp] = u;
	inst[u] = 1;
	for(int v : vec[u]) {
		if(!dfn[v]) {
			tarjan(v);
			low[u] = min(low[u], low[v]);
		} else if(inst[v]) {
			low[u] = min(low[u], dfn[v]);
		}
	}
	if(dfn[u] == low[u]) {
		sc ++;
//--------------------------------
		while(st[tp] != u) {
			scc[st[tp]] = sc;
			sz[sc] ++;
			tp --;
		}
		scc[u] = sc;
		sz[sc] ++;
		tp --;
	}
//--------------------------------
//  这一段忘记了 inst[st[tp]] = 0;
//  依然可以过
//--------------------------------
	return ;
} 
int main() {
	cin >> n >> m;
	for(int i = 1; i <= m; i ++) {
		int a, b;
		cin >> a >> b;
		vec[a].push_back(b);
	}
	for(int i = 1; i <= n; i ++) {
		if(!dfn[i]) {
			tarjan(i);
		}
	}
	for(int i = 1; i <= n; i ++) {
		for(int j : vec[i]) {
			if(scc[i] == scc[j]) continue;
			out[scc[i]] ++;
		}
	}
	bool f = 0;
	int ans = 0;
	for(int i = 1; i <= sc; i ++) {
		if(out[i] == 0) {
			if(f) {
				ans = 0;
				break;
			}
			f = 1;
			ans = sz[i];
		}
	}
	cout << ans << "\n";
	return 0;
}