快速幂 fast_exp

long long fast_exp(int base,long long exp,int mod)
{
    long long ans=1LL,a=base;
    while(exp!=0)
    {
        if(exp&1LL) ans*=a,ans%=mod;
        a*=a,a%=mod;
        exp>>=1;
    }
    return ans%mod;
}