simple sieve

//simple sieve
bitset<N> arr;ll prime[N],p;

void sieve(int n) {
    for(int i = 0; i <= n; i++) arr[i] = 0;
    double z = sqrt(n);
    arr[0] = arr[1] = 1;
    for(int i = 4; i <= n; i += 2) arr[i] = 1;
    for(int i = 3; i <= z; i += 2) {
        if(arr[i] == 0)
        for(ll j = i*i; j <= n; j += i + i) {
            arr[j] = 1;
        }
    }
    prime[p++] = 2;
    for(int i = 3; i <= n; i += 2) if(!arr[i]) prime[p++] = i;
}