Hi, I'm the original author of this Kata (Ruby version). Author of the haskell version didn't understand this Kata properly, and indeed added a DFS version instead of BFS.
I have learned haskell to fix this. I added a BFS solution and a correct buildTree (thanks to @nickie for providing one). Please take a look again.
I think it's so clear! Do you know typo rules! there is always a space after a comma or a point!
Anything else beside the typo in "decreasing"?