ac:

#include <bits/stdc++.h>
using namespace std;
bool a[100000001];

int main() {

	long long n, ans = 0, ens;
	cin >> n;
	for (long long i = 2; i <= n; i++) {
		if (a[i] == 0) {
			ans++;
			ens = 2;
			for (long long j = i; j * ens <= n; ens++) {
				a[j * ens] = 1;
			}

		}

	}
	cout << ans;
	return 0;
}

直接报错：

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

int main() {
bool a[10000001];

	long long n, ans = 0, ens;
	cin >> n;
	for (long long i = 2; i <= n; i++) {
		if (a[i] == 0) {
			ans++;
			ens = 2;
			for (long long j = i; j * ens <= n; ens++) {
				a[j * ens] = 1;
			}

		}

	}
	cout << ans;
	return 0;
}