Code mẫu

#include <bits/stdc++.h>
using namespace std;

const int N = 200005;

int n, m, u, v, chk[N];
int cyc, dp[2 * N][2];
vector<int> stk, adj[N], bip[2 * N];

void DFS_1(int u) {
    chk[u] = 1;
    stk.push_back(u);
    for (int v : adj[u]) {
        if (chk[v] == 0) {
            DFS_1(v);
        } else if (chk[v] == 1) {
            cyc++;
            bip[v].push_back(cyc);
            bip[cyc].push_back(v);
            for (int i = stk.size() - 1; i >= 0; i--) {
                if (stk[i] == v) {
                    break;
                } else {
                    bip[stk[i]].push_back(cyc);
                    bip[cyc].push_back(stk[i]);
                }
            }
        }
    }
    chk[u] = 2;
    stk.pop_back();
}

void DFS_2(int u, int p = 0) {
    int mi = n;
    for (int v : bip[u]) {
        if (v != p) {
            DFS_2(v, u);
            if (u <= n) {
                // original node
                dp[u][0] += dp[v][1];
                dp[u][1] += min(dp[v][0], dp[v][1]);
            } else {
                // cycle node
                dp[u][0] += min(dp[v][0], dp[v][1]);
                dp[u][1] += min(dp[v][0], dp[v][1]);
                mi = min(mi, max(0, dp[v][1] - dp[v][0]));
            }
        }
    }
    dp[u][1] += (u <= n ? 1 : mi);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    // freopen("RESTRICTIONS.inp", "r", stdin);
    // freopen("RESTRICTIONS.out", "w", stdout);
    cin >> n >> m;
    cyc = n;
    for (int i = 1; i <= m; i++) {
        cin >> u >> v;
        adj[u].push_back(v);
    }
    for (int i = 1; i <= n; i++) {
        if (chk[i] == 0) {
            DFS_1(i);
        }
    }
    int ans = 0;
    for (int i = n + 1; i <= cyc; i++) {
        if (dp[i][1] == 0) {
            DFS_2(i);
            ans += dp[i][1];
        }
    }
    cout << n - ans;
}