HDU 4990 Reading comprehension

快速幂

#include<cstdio>

#include<cstring>

#include<cmath>

#include<iostream>

#include<algorithm>

using namespace std;



long long n,MOD;



long long cal(long long a,long long b,long long mod)

{

    long long c=1;

    while(b!=0)

    {

        if(b%2==1) c=(c*a)%mod,b--;

        else a=(a*a)%mod,b=b/2;

    }

    return c;

}



int main()

{

    while(cin>>n>>MOD)

    {

        if(n%2==1)

            cout<<(cal(2,n+1,MOD*3)-1)/3<<endl;

        else

            cout<<(cal(2,n+1,MOD*3)-2)/3<<endl;

    }

    return 0;

}