素數篩

 for(long long i = 2; i <= 1010000; i += 2)
        a[i] = 0;
    b[1] = 2;
    for(long long i = 3; i <= 1010000; i += 2)
    {
        if(a[i])
        {
            b[t++] = i;
            for(long long j = i * i; j <= 1010000; j += i * 2)      // j = i * i, 因爲i乘以比i小的都被之前篩掉
            {
                a[j] = 0;
            }
        }
    }