Using Recursion to reverse an integer without the use of strings

Issue

I have been trying this for some time now but could not get it to work. I am trying to have a method to reverse an integer without the use of strings or arrays. For example, 123 should reverse to 321 in integer form.

My first attempt:

/** reverses digits of integer using recursion */
public int RevDigs(int input)
{
    int reverse = 0;
    if(input == 0)
    {
        return reverse;
    }
    int tempRev = RevDigs(input/10);
    if(tempRev >= 10)
        reverse = input%10 * (int)Math.pow(tempRev/10, 2) + tempRev;
    if(tempRev <10 && tempRev >0)
        reverse = input%10*10 + tempRev;
    if(tempRev == 0)
        reverse = input%10;   
    return reverse;
}//======================

I also tried to use this, but it seems to mess up middle digits:

/** reverses digits of integer using recursion */
public int RevDigs(int input)
{
    int reverse = 0;
    if(input == 0)
    {
        return reverse;
    }
    if(RevDigs(input/10) == 0)
        reverse = input % 10;
    else
    {
        if(RevDigs(input/10) < 10)
            reverse = (input % 10) *10 + RevDigs(input/10);
        else
            reverse = (input % 10)* 10 * (RevDigs(input/10)/10 + 1) + RevDigs(input/10);
        }
    return reverse;
}

I have tried looking at some examples on the site, however I could not get them to work properly. To further clarify, I cannot use a String, or array for this project, and must use recursion. Could someone please help me to fix the problem. Thank you.

Solution

What about:

public int RevDigs(int input) {
    if(input < 10) {
        return input;
    }
    else {
        return (input % 10) * (int) Math.pow(10, (int) Math.log10(input)) + RevDigs(input/10);
        /* here we:
           - take last digit of input
           - multiply by an adequate power of ten
             (to set this digit in a "right place" of result)
           - add input without last digit, reversed
        */
    }
}

This assumes input >= 0, of course.

Answered By - Michał Rybak