Project Euler Problem 38

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Problem 38 : Pandigital multiples

Take the number 192 and multiply it by each of 1, 2, and 3:

192 × 1 = 192
192 × 2 = 384
192 × 3 = 576
By concatenating each product we get the 1 to 9 pandigital, 192384576. We will call 192384576 the concatenated product of 192 and (1,2,3)

The same can be achieved by starting with 9 and multiplying by 1, 2, 3, 4, and 5, giving the pandigital, 918273645, which is the concatenated product of 9 and (1,2,3,4,5).

What is the largest 1 to 9 pandigital 9-digit number that can be formed as the concatenated product of an integer with (1,2, … , n) where n > 1?

#include <iostream>
#include <ctime>
#include <algorithm>

using namespace std;

// #define UNIT_TEST

class PE0038
{
private:
    int m_digits[6];
    int m_productDigits[16];
    int m_numOfProductDigits;

    int  getDigits(int number);
    bool checkValidPandigital(const int productDigits[]);
    bool checkPandigital(int number, int max_value);

public:
    long int findLargestConcatenatingPandigital();
};

int PE0038::getDigits(int number)
{
    int numOfDigits = 0;

    while(number> 0)
    {
        m_digits[numOfDigits++] = number%10;
        number /= 10; 
    }

    return numOfDigits;
}

bool PE0038::checkValidPandigital(const int productDigits[])
{
    for(int i=0; i<9; i++)
    {
        if (productDigits[i] != i+1)
        {
            return false;
        }
    }

    return true;
}

bool PE0038::checkPandigital(int number, int max_multiplier)
{
    int product;

    m_numOfProductDigits = 0;

    for(int i=1; i<=max_multiplier; i++)
    {
        product = number * i;

        int numOfDigits = getDigits(product);
        
        for(int i=numOfDigits-1; i>=0; i--)
        {
            m_productDigits[m_numOfProductDigits++] = m_digits[i];
        }
    }

    if (9 == m_numOfProductDigits)
    {
        int tmpProductDigits[9];
        memcpy(tmpProductDigits, m_productDigits, 9*sizeof(int));

        sort(tmpProductDigits, tmpProductDigits+9);

        if (true == checkValidPandigital(tmpProductDigits))
        {
#ifdef UNIT_TEST
            cout << "pandigital: " << number << " (1.." << max_multiplier << ") ";
            for(int i=0; i<m_numOfProductDigits; i++)
            {
                cout << m_productDigits[i]; 
            }
            cout << endl;
#endif
            return true;
        }
    }

    return false;
}

long int PE0038::findLargestConcatenatingPandigital()
{
    long int value;
    long int largest_value = 0;
    int largest_n, largest_number;
    int max_number = 10000; //max number 10000 when n=2

    for (int number=9;number<max_number; number++)
    {
        for (int n=2;n<=5;n++)
        {
            if (true == checkPandigital(number, n))
            {
                value = 0;
                for(int i=0; i<9; i++)
                {
                    value = 10*value + m_productDigits[i];
                }
                if (largest_value < value)
                {
                    largest_value  = value;
                    largest_n      = n;
                    largest_number = number; 
                }
            }
        }
    }

#ifdef UNIT_TEST
    cout << "The number " << largest_number << " is multiplied by each of " << endl;
    cout << "(1,...," << largest_n <<") and the concatenated pandigital is ";
    cout << largest_value << endl;
#endif

    return largest_value;
}

int main()
{
    clock_t start = clock();

    PE0038 pe0038;

    cout << pe0038.findLargestConcatenatingPandigital() << " is the largest 1 to 9 " ;
    cout << "pandigital 9-digit number " << endl; 
    cout << "that can be formed as the concatenated product of an integer " << endl;
    cout << "with (1,2, ... , n) where n > 1" << endl;

    clock_t finish = clock();
    double duration = (double)(finish - start) / CLOCKS_PER_SEC;
    cout << "C/C++ application running time: " << duration << " seconds" << endl;

    return 0;
}