URAL 1226 Little Chu（最大原根）

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
#include <climits>
#include <cmath>
#include <ctime>
#include <cassert>
#include <bitset>
#define IOS ios_base::sync_with_stdio(0); cin.tie(0);
using namespace std;
typedef long long ll;
const int MAX_N = 70000;

int T, n, prime_cnt, factor_cnt;
int prime[MAX_N], factor[MAX_N];
bitset<MAX_N> bs;

void GetPrime()
{
bs.set();
prime_cnt = 0;
for(int i = 2; i < MAX_N; i++){
if(bs[i] == 1) { prime[prime_cnt++] = i; }
for(int j = 0; j < prime_cnt && i * prime[j] < MAX_N; j++){
bs[i * prime[j]] = 0;
if(i % prime[j] == 0) break;
}
}
}

void GetFactor(int x)
{
factor_cnt = 0;
for(int i = 0; i < prime_cnt && prime[i] * prime[i] <= x; i++){
if(x % prime[i] == 0){
factor[factor_cnt++] = prime[i];
while(x % prime[i] == 0){
x /= prime[i];
}
}
}
if(x > 1){
factor[factor_cnt++] = x;
}
}

int quick_pow(int a, int b, int m)
{
ll res = 1, tmp = (ll)a, mod = (ll)m;
while(b){
if(b & 1) res = res * tmp % mod;
tmp = tmp * tmp % mod;
b >>= 1;
}
return (int)res;
}

int main()
{
GetPrime();
scanf("%d", &T);
while(T--){
scanf("%d", &n);
GetFactor(n - 1);
int ans ;
for(int i = n - 1; i >= 2; i--){
int flag = true;
for(int j = 0; j < factor_cnt; j++){
if(quick_pow(i, (n - 1) / factor[j], n) == 1){
flag = false;
//printf("i = %d factor[j] = %d\n", i, factor[j]);
break;
}
}
if(flag){
ans = i;
break;
}
}
printf("%d\n", ans);
}
return 0;
}