Half memory seive

const int MX = 5e4+123;
bool seive[MX];
vi prime;

void seiveGen(int limit) {
    limit += 100;
    int sqrtn = sqrt(limit);
    for(int i = 3; i <= sqrtn; i += 2) {
        if(!seive[i>>1]) {
            for(int j = i * i; j < limit; j += i + i) {
                seive[j>>1] = 1;
            }
        }
    }

    prime.PB(2);
    for(int i = 3; i < limit; i += 2) {
        if(!seive[i>>1]) prime.PB(i);
    }
}