赛前写一下

GCD

int gcd(int a, int b)
{
    return b ? gcd(b, a % b) : a;
}

LCM

int lcm(int a, int b)
{
    return a / gcd(a, b) * b;
}

位运算

int lowbit(int x)
{
    return x & -x;
}

int main()
{
    cin >> x;
    int res = 0;
    while (x)
    {
        x -= lowbit(x);
        res ++;
    }

}

线性筛

void get_primes()
{
    for (int i = 2;i <= n; i ++)
    {
        if (!st[i])
            primes[cnt ++] = i;

        for (int j = 0; primes[j] <= n / i; j ++)
        {
            st[primes[j] * i] = true;
            if (i % primes[j] == 0) break;
        }
    }
}