your approach is totally wrong. Not an issue.

Description updated

looks like java is not your language
StackOverflowError :)

When no upper bound is given, you should assume the numbers may run up to an indefinite number of digits.

No.

After 1e5, it asks for one more result, zeros(1e9).

Your solution has to be O(log n), not O(n).

In Java, test4 is the only one not passing for me. It doesn't even say what the result is supposed to be. I've tried debugging it by printing out the arrays, and even printing out the number of elements in the arrays. Both of them aren't showing anything. What am I doing wrong?

This makes me so mad lol great job.

It would have been a little more helpful if the kata description specified that the numbers can use more than 7 bits, but great kata.

Can this kata be solved using recursion? And if so, is there anything I should keep in mind? My code works perfectly fine in my IDE, but on here it's giving me wrong answers.

Tell me about it.

I figured out what's going on. In the random test cases, it's appending 'u' after question marks. I'm confused as to why it does that because I already specified a question mark as a case in which it should just be appended to the final string, and not even go through the process of appending vowels. Is a question mark like some weird operator or something?