Well, just the regular concat (+ sign, I mean).

If you're looking for a way to optimize, changing the way to combine the chars is not the way to go. Instead, search for a way to reduce your loop.

Whad did you use to cocnatination? Just +, stringbuilder or StreamAPI? Please give me advice

It's real ;)

Anyone had problem with Java? 'System.out.print' make succesful answer, but when I try return 'String' with answer 'Time limit exceed'. Why?

Hi, is that real on java?

Not an issue, use System.out.println. Also, you probably should read the section of the docs about training on kata.

Cheers

Java17, 2 start test complete, other 3 NULLPOINTEREXCEPTION, what te hell??? Let me even see what commands do you enter in this class