Thanks for nice kata! Looked much easier on the first sight than it really was (expected 6th kyu at least), especially because of the array visulization issues. In Unity3D it would be piece of cake but I didn't want this shortcut and was lazy to vizualize it using some temporary string visualization method (which turned out such a laziness was a big mistake). This is kind of scanario which would be much more difficult for me without TDD safety.

It's a lambda expression! Sort of like a simplified method. In this case, you can think of the => as a kind of return statement.

What does the => do?

how was it possible?

Nice one

Thanks for this great kata! I really enjoyed this one a lot. TDD shines here like a diamond. Everybody who says TDD or general unit testing is waste of time should try to solve this kata without tests...

The instructions state you won't get negative numbers on input, therefore IMO it's like a contract and you really don't need to count with negative numbers in this case. This is why the kata tests also pass because they won't send you any single negative number. Also check the similar answer from @Chrono79 below.

The exercise instructions clearly state this, which makes it a requirement.

This is correct since the acceptance criteria don't expect negative numbers as input parameter. We are not trying to do classic sad path validation.

Not exists validation for negative numbers

Wrong decision