埃氏素数筛 On

const int maxn = 1e+6 + 7;
bool prime[maxn];
int rec[maxn], cnt;
void init_prime()
{
    cnt = 0;
    memset(prime, true, sizeof(prime));
    prime[0] = prime[1] = false;///表明true为质数
    for (int i = 2; i <= maxn; ++i){
        if (prime[i]) rec[cnt++] = i;
        //此处边界判断为rec[j] <= maxn / i,如果写成i * rec[j] <= maxn,需要确保i * rec[j]不会溢出int
        for (int j = 0; j < cnt && rec[j] <= maxn / i; ++j){
            prime[i * rec[j]] = false;
            if (i % rec[j] == 0)
                break;
        }
    }
}