7 контест 2 задача

#include <iostream>
#include <vector>

using namespace std;

vector<vector<int>> g, g_r;
vector<bool> used;
vector<int> order;
vector<int> component;

void dfs(int v) {
    used[v] = true;
    for (int i : g[v])
        if (!used[i])
            dfs(i);
    order.emplace_back(v);
}

void dfs_r(int v) {
    used[v] = true;
    component.push_back(v);
    for (int i : g_r[v])
        if (!used[i])
            dfs_r(i);
}

int main() {
    int n, m;
    cin >> n >> m;

    used = vector<bool> (n);
    g = vector<vector<int>> (n);
    g_r = vector<vector<int>> (n);
    vector<pair<int,int>> s(m);

    for (int i = 0; i < m; i++) {
        int fr, to;
        cin >> fr >> to;
        fr--; to--;
        s[i] = make_pair(fr, to);
        g[fr].emplace_back(to);
        g_r[to].emplace_back(fr);
    }

    for (int i = 0; i < n; ++i)
        if (!used[i])
            dfs(i);

    used = vector<bool> (n);

    vector<int> component_per_v(n);
    int cnt = 0;
    for (int i = 0; i < n; ++i) {
        int v = order[n - 1 - i];
        if (!used[v]) {
            dfs_r(v);
            for (auto j : component){
                component_per_v[j] = cnt;
            }
            cnt++;
            component.clear();
        }
    }

    vector<bool> ans(cnt);
    for (int i = 0; i < m; i++){
        if (component_per_v[s[i].first] != component_per_v[s[i].second]){
            ans[component_per_v[s[i].first]] = true;
        }
    }

    vector<int> f_ans;
    for (int i = 0; i < cnt; i++){
        if (!ans[i]){
            for (int j = 0; j < n; j++){
                if (component_per_v[j] == i){
                    f_ans.emplace_back(j);
                    break;
                }
            }
        }
    }
    cout << f_ans.size() << '\n';
    for (auto &i : f_ans){
        cout << i + 1 << '\n';
    }
}