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!

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.