You're quite right. I used string too, before passing all tests. The reason, in my opinion, is that string catenation involves repetitive memory allocation 'coz a new string is created each time. But for byte slice, RAM is much more efficient, therefore faster.

Think twice before cut once.

That's true, REALLY.

Agree~

I admit, I learnt a lot from this kata~

I can only think that: it's designed this way, on purpose~

Yeah, agree~

Same situation here. Hope author will fix it soon.