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.

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.

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.

Check my recursive solution if interested. Similar approach but I needed extra method (to be able to pass list as parameter).

Risky business as non English characters like 'č', 'ř', 'ž' will break the logic. Char.IsLetter('č') returns true.

Excellent kata, love it!
I only had some problems with mocking IEngine, IFuelTank, IFuelTankDisplay while testing car class so I created another public constructor there which received all 3 interfaces / mocks.
Great exercise.

Nope, input states 1000 ≤ year ≤ 9000. Rule in TDD is to satisfy the test in the easiest possible way.

Nope, Input states 1000 ≤ year ≤ 9000.
The solution is correct and tests are green.
Why to create plane if customer just wanted regular car?

Thanks for this great kata. Love it!

Bruno, this series is awesome! A lot of fun. Thanks

Nice solutions guys. I just couldn't see the math behind it so I chose the hard way (kind of 5kyu kata in this way of 60 lines of code ;) ).

Thanks for the great Kata. It was plenty of fun. Next time I'd try to do it thread safe.

When I saw the top solution, I was kind of embarassed ;). Nice one!