Cod sursa(job #1284596)

Utilizator felixiPuscasu Felix felixi Data 6 decembrie 2014 17:18:22
Problema Invers modular Scor 100
Compilator cpp Status done
Runda Arhiva educationala Marime 1.02 kb
#include <fstream>
#include <cmath>

using namespace std;

ifstream in("inversmodular.in");
ofstream out("inversmodular.out");

int phi(int n)
{
    int lim=(int)sqrt((double)n);
    int ans=n;
    for(int i=2; i<=lim; i++)
    {
        bool ok=false;
        while(n%i==0)
        {
            n/=i;
            ok=true;
        }
        if(ok)
        {
            ans/=i;
            ans*=(i-1);
            lim=(int)sqrt((double)n);
        }
    }
    if(n>1)ans=ans/n*(n-1);
    return ans;
}
int lgput(int n, int p, int mod)
{
    long long x=n;
    long long power=1;
    for(unsigned int i=0; (1LL<<i)<=p; i++)
    {
        if(((1<<i)&p))
            power=(power*x)%mod;
        x=(x*x)%mod;
    }
    return power;
}

int Inv_mod(int a, int n) {
    return lgput(a, phi(n)-1, n);
}

int main()
{
    freopen("inversmodular.in", "r", stdin);
    freopen("inversmodular.out", "w", stdout);
    int a, n;
    in >> a >> n;
    out << Inv_mod( a, n ) << '\n';
    return 0;
}