老哥回复被吞了，给老哥写了个欧拉筛，直接怼面试官 #include <iostream> using namespace std; const int N = 1000010; int cnt,n; int primes[N]; bool st[N]; int main() { cin >> n; for (int i = 2; i <= n; i++) { if (!st[i]) primes[cnt++] = i; for (int j = 0; primes[j] <= n/i; j++) { st[i * primes[j]] = true; if (i % primes[j] == 0) break; } } cout << cnt << endl; return 0; }