51nod 1135 原根 （数论）

#include<bits/stdc++.h>
using namespace std;
int p[100010];
int t = 0;
//http://blog.csdn.net/acdreamers/article/details/8883285
void divide(long long n)
{
for(int i = 2; i * i <= n; i++){
if(n % i == 0){
p[t ++] = i;
while(n % i == 0) n /= i;
}
}
}

int solve(long long x,long long n,long long m)
{
long long res = 1;
while(n > 0){
if(n & 1) res = (res * x) % m;
x = (x * x) % m;
n >>= 1;
}
return res;
}

int main()
{
long long a;
cin >> a;
divide(a - 1);//求a-1所有的质因子
for(int i = 2; i < a; i++)
{
bool flag = false;
for(int j = 0; j < t; j++)
{
if(solve(i,(a - 1) / p[j], a ) == 1)//判断是否是原根
{
flag = true;
break;
}
}
if(!flag){
cout << i << endl;
break;
}
}
return 0;
}