c++ rearranging an integer

Question

I'm currently a C++ student and I'm on the basics part of C++, so please excuse me if this seems simple for you guys but I find it the hardest problem I ever had: It is given a number n with exactly 8 digits. Find the biggest palindrome number obtained by the rearrangement of all digits of n. Example: 21523531 => 53211235

I know vectors and how to check if a number is palindrome, I also know how to remove each digit from a number until it gives me a palindrome number, but I have no ideea how I can rearrange ALL digits.

Answer 1

I think followng code will work:

int func(int x) // returns 0 if there is no valid palindrome
{
    int arr[10]{};
    while (x)
    {
        arr[x%10]++;
        x /= 10;
    }
    for (int it : arr)
        if (it % 2)
            return 0;
    int ret = 0;
    for (int i = 9; i >= 0; i--)
        for (int j = 0; j < arr[i]/2; j++)
            ret = ret * 10 + i;
    for (int i = 0; i < 10; i++)
        for (int j = 0; j < arr[i]/2; j++)
            ret = ret * 10 + i;
    return ret;
}

And here is more student-readable version:

int func(int x) // returns 0 if there is no valid palindrome
{
    int arr[10] = {0,0,0,0,0,0,0,0,0,0}; // it is digits counter
    while (x) // this is executed for each digit in the number
    {
        arr[x%10]++; // here x%10 is equal to current digit
        x /= 10; // remove rightmost digit
    }
    for (int i = 0; i < 10; i++)
    {
        if (arr[i] % 2) // Every digit must be repeated 0,2,4,6 or 8 times. If it's not, then there is no valid palindrome
        {
            return 0;
        }
    }
    int ret = 0; // this is our future result
    // Now we just take largest digit that appears in source number and add it to the result. If it appears twice, we add it once. If it appears 4 times, we add it twice and so on.
    for (int i = 9; i >= 0; i--)
    {
        for (int j = 0; j < arr[i]/2; j++)
        {
            ret = ret * 10 + i;
        }
    }
    // Now we're doing same thing in the reverse direction.
    for (int i = 0; i < 10; i++)
    {
        for (int j = 0; j < arr[i]/2; j++)
        {
            ret = ret * 10 + i;
        }
    }
    return ret; // we're done now
}

I hope you'll find this answer usefull and will use it without blind copy.