Euler’s Totient function, φ(n) [sometimes called the phi function], is used to determine the number of positive numbers less than or equal to n which are relatively prime to n. For example, as 1, 2, 4, 5, 7, and 8, are all less than nine and relatively prime to nine, φ(9)=6.
The number 1 is considered to be relatively prime to every positive number, so φ(1)=1.

Interestingly, φ(87109)=79180, and it can be seen that 87109 is a permutation of 79180.

Find the value of n, 1 < n < 107, for which φ(n) is a permutation of n and the ratio n/φ(n) produces a minimum.

參考程式碼：

#include <iostream>
using namespace std;

#define MAX 10000
#define TAR 10000000

bool state[MAX] = { 0
 
 };
int pri[MAX] = { 0 };
int cnt = 0;

double check(int im, int in)
{
    int i, m = im, n = in;
    int cnt1[10] = { 0 };
    int cnt2[10] = { 0 };
    while (m > 0) {
        cnt1[m % 10]++;
        m /= 10;
    }
    while (n > 0) {
        cnt2[n % 10]++;
        n /= 10;
    }
    for (i = 0; i < 10; i++) {
        if
 
 (cnt1[i] != cnt2[i]) {
            break;
        }
    }
    if (i == 10) {
        return (double)im / in;
    }else {
        return TAR;
    }
}

int main()
{
    double result = TAR;
    int n = 0;
    for (int i = 2; i < MAX; i++) {
        state[i] = 1;
        pri[i] = 0;
    }
    for (int i = 2; i < MAX; i++) {
        if (state[i] == 1) {
            for (int j = 2 * i; j < MAX; j += i) {
                state[j] = 0;
            }
            pri[cnt] = i;
            cnt++;
        }
    }
    for (int i = 2; i < cnt; i++) {
        for (int j = i + 1; j < cnt; j++) {
            if (pri[i] * pri[j] < TAR) {
                double temp = check(pri[i] * pri[j], (pri[i] - 1)*(pri[j] - 1));
                if (temp < result) {
                    result = temp;
                    n = pri[i] * pri[j];
                }
            }
        }
    }
    cout << n << endl;
    return 0;
}