快速幂

快速幂，a的b次方；

LL sm(LL a, LL b)
{
    LL ans = 1;
    a = a%mod;
    while (b > 0)
    {
        if (b % 2 == 1)
            ans = (ans*a) % mod;
        b = b / 2;
        a = (a*a) % mod;
    }
    return ans%mod;
}