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 :).

Solution depends on case. Should not pass tests.

you can create stream of Characters initially to avoid additional boxing. (check my iml)

Ha! This is realy clever!

@xehpuk, you are correct, my mistake, thanks!

What concatenation? This solution uses basic arithmetic. Simple and performant.

It's bad practice! Because every use of the substring method creates a new string. Hence we pollute the string pool. See mine....

But the use of concatenation in loops (or recursions) is an awful practice. Should remember during daily work.