The best and understandable

Great!

What's wrong with my code?

I guess it solution is similiar to mine and it looks good. I have only one problem with that solution that you called integer from array that you are using "i". I don't think you should call array item "i" because it can be confusing. But still good job!

As I read the instructions, it's supposed to give UPPERCASE responses. But if it works.

Does it?

makes sense ty

In this case as in any other case I think it is sensible to avoid nesting loops.
Nested loops make for hard to understand code and do push your algorithm into a higher complexity. Depending on the problem at hand (large n) this can be a deal breaker.

wow! adding "_" to s at beginning is really smart! so we don't need to check if the s.length() is odd or even. Thank you for your solotion. It's easy to understand.

Good solution!