Thanks for releasing interesting missions.

I solved all but the following tests.

assert riichi_mahjong_shanten('dw', 'dr', 'we', 'dg', 's1', 'ww', 'm9', 'wn', 'm1', 'p1', 's9', 'ws', 'ha', 'p9') == 1

It contains invalid tile: 'ha'.

I think it would be a better mission to fix the parameters to either 14 or 13 tiles list. Because parameter testing is not the essence of the mission.