LS 38 Geometric sum（数论+二分快速幂）

Geometric sum

Input

Output

Sample input

2 2 1000000000

Sample output

6

#include<iostream>
#include<cstring>
using namespace std;
long long c,x,n;
long long mul(long long a,long long b)///(a*b)%c
{ long long ret=0;
a%=c;b%=c;
while(a)
{
if(a&1)
ret=(ret+b)%c;
a>>=1;b<<=1;b%=c;
}
return ret;
}
long long sqr(long long x,long long n)///(x^n)%c
{ long long ret=1;
if(n==1)return x%c;
while(n)
{if(n&1)ret=mul(x,ret);
x=mul(x,x);n>>=1;
}
return ret;
}
long long pow(long long x,long long n)///(x+x^2...)%c
{ long long ret=0;
if(n==1)return x%c;
if(n&1)
{
ret=mul(pow(x,n/2),sqr(x,n/2+1L)+x);
ret=(ret+x)%c;
}
else
{
ret=mul(pow(x,n/2),sqr(x,n/2)+1L);
}
return ret;
}
int main()
{
while(cin>>x>>n>>c)
{
cout<<pow(x,n)<<"\n";
}
}