How to change a number's digits in a recursive function? C++

Question

I have to input a number n, an a digit and a b digit and output the number n with all the a digits in it replaced by a b one. For example:

Input:
n = 1561525
a = 5
b = 9

Output:
n = 1961929

Should be recursive ! I didn't post any code as I've done it in a non-recursive way but apparently it's not even close to what I need.

Thanks for the help !

Answer 1

Check this, it works but maybe it is to much C

int convert(int num, int a, int b)
{
    if( num )
    {
        int res = convert(num/10,a,b);
        int t = num%10;
        res *=10;
        t = t == a ? b:t;
        res = res + t;
        return res;
    }
    return 0;
}

Divide by 10 the initial number, until nothing left of it, and then construct it again replacing a with b.