rt，，错的点都输出了 0，感觉少了 conner case

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e6 + 5;

int n, m;
vector<int> e[N];
int dfn[N], low[N], tim, scc_cnt;
bool ins[N];
stack<int> stk;
int siz[N], bel[N];
struct graph {
	vector<int> g[N];
	int in_deg[N], f[N];
	int sum;
	void addedge(int u, int v) {
		g[u].push_back(v);
		in_deg[v] ++;
	}

	void sol() {
		queue<int> q;
		for (int i = 1; i <= n; i ++) {
			if (!in_deg[i]) {
				q.push(i);
			}
		}
		while (q.size()) {
			int u = q.front();
			q.pop();
			sum --;
			if (q.size()) {
				f[u] = -1e9;
			} else {
				f[u] = sum;
			}
			for (auto v : g[u]) {
				if (!-- in_deg[v]) {
					q.push(v);
				}
			}
		}
	}

} G1, G2;

void tarjan(int u) {
	dfn[u] = low[u] = ++ tim;
	stk.push(u), ins[u] = true;
	for (auto v : e[u]) {
		if (!dfn[v]) {
			tarjan(v);
			low[u] = min(low[u], low[v]);
		} else if (ins[v]) {
			low[u] = min(low[u], dfn[v]);
		}
	}
	if (dfn[u] == low[u]) {
		int v;
		scc_cnt ++;
		do {
			v = stk.top(), stk.pop();
			ins[v] = false;
			bel[v] = scc_cnt, siz[scc_cnt] ++;
		} while (v != u);
	}
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr); cout.tie(nullptr);

    cin >> n >> m;
    for (int i = 1; i <= m; i ++) {
    	int u, v;
    	cin >> u >> v;
    	e[u].push_back(v);
    }
    for (int i = 1; i <= n; i ++) {
    	if (!dfn[i]) {
    		tarjan(i);
    	}
    }


    for (int u = 1; u <= n; u ++) {
    	for (auto v : e[u]) {
    		if (bel[u] == bel[v]) {
    			continue;
    		}
    		G1.addedge(bel[u], bel[v]);
    		G2.addedge(bel[v], bel[u]);
    	}
    }
    G1.sum = G2.sum = scc_cnt;
    G1.sol(), G2.sol();

    int ans = 0;
    for (int i = 1; i <= scc_cnt; i ++) {
    	if (G1.f[i] + G2.f[i] == scc_cnt - 1) {
    		ans += siz[i];
    	}
    }

    cout << ans << "\n";

    return 0;
}