That's a common feeling, it's okay to feel that. Just remember: you aren't stupid, and you weren't. Sometimes a one's mind can play tricks and block a way to the solution, even the simplest soulutions. But now you've made it, you've learned something new. Cheers! :)

i feel so stupid right now

this is acc realy easy to read ,why would we need to debug this

I think your solution is way less efective, and in this top rated solution is problem with creating String s (it's redundant) we should just return new string from substring and that is it. Clear effictient and without redundancy :).

Nice to debug :)