Agree, I don't think this solution the best practice.

Its easily a 2 kyu

Hi IceGlance, thanks for your compliment! You described exactly what the purpose is of having 2 kata's: the 4x4 can be solved brute force while that approach might not work on the 6x6 because of the exponential amount of possibilities.

It looks like some of the base grids used to generate unsolvable cases might accidentally generate valid case. :/ That's annoying...

Does it happen often?

edit: are you absolutely sure this is the input causing troubles? (note: I checked, this one is effectively solvable)