站外题求助

https://www.luogu.com.cn/problem/U505175

juruo 的代码：

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

#define LL long long
#define Pii pair<int, int>
#define ULL unsigned long long

namespace gdb7
{
    bool goodnum[10000010], is_prime[10000010], cnt;
    int prime[1000010];

    int main() {
        int n;
        cin >> n;
        is_prime[0] = is_prime[1] = true;
        for (int i = 2; i <= n; ++i) {
            if (!is_prime[i]) {
//              cout << "    i" << i << endl;
                prime[++cnt] = i;
                for (LL j = 1; j <= n; ++j) {
                    goodnum[j] = true;
                }
            }
            for (int j = 1; j <= cnt && i * prime[j] <= n; ++j) {
                is_prime[i * prime[j]] = true;
                if (i % prime[j] == 0) {
                    break;
                }
            }
        }
        for (int i = 1; i <= n; ++i) {
            cout << "("<< is_prime[i] << ", " << goodnum[i] << "), ";
        }
        cout << "\ncnt:" << cnt << endl;
        for (int i = 1; i <= cnt; ++i) {
            cout << prime[i] << " ";
        }
        cout << endl;
        for (int i = n; i >= 1; --i) {
            if (goodnum[i]) {
                cout << i << endl;
                break;
            }
        } 
        return 0;
    }
};

int main()
{
    return gdb7::main();
}