Problem #41

This one was actually quite easy.

#include <iostream>
#include <string>
#include <ctime>
#include <cmath>
#include <algorithm>

using namespace std;

bool isPrime (int);
 
int main(void) 
{
    clock_t start = clock();
    int total = 0;
    string str;
    int holder[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    int holder2 = 0;

    for (int j = 1; j <= 9; j++)
    {
        sort (holder, holder + j);

        do {
            str.clear();
            for (unsigned int i = 0; i < j; i++)
                str += holder[i] + '0';
            holder2 = atoi (str.c_str());
            if (isPrime (holder2))
                if (holder2 > total)
                    total = holder2;
        } while ( next_permutation (holder,holder+j) );
    }


    cout << total << endl 
         << "Process took " << (static_cast<double> (clock()) - start) / CLOCKS_PER_SEC << " seconds." << endl;
    system("pause");
    return 0;
}

bool isPrime (int num)
{
    if (num == 0 || num == 1)
        return false;
  
    if (num == 2 || num == 3)
        return true;

    for (unsigned int i = 2; i < static_cast<int> (sqrt (static_cast<double> (num))) + 1; i++)
        if (num % i == 0)
            return false;
   return true;
}