• Initial code (and reference solution) is completely wrong and won't even compile
• Lack of tests
• The same concept has many duplicates: it's just StringBuilder ;-)
• Even with that said, join or * will work just as fast so nothing is enforced

If you want to force a perfomant solution, you should test it in a way trivial solution doesn't work.
Typo resaons.
Only one test is not enough either.

I can't even pass the tests, they expect something different from n dots.