Left and right view of binary tree

left_right_view_bt.hs

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-- https://gist.github.com/evgenii-malov/1fc29a652751451dbca0d54d454cc1ef
import PrettyT

-- data Btree a = Empty | Node a (Btree a) (Btree a) deriving Show


paths :: Btree a -> [[a]]
paths Empty = []
paths (Node a Empty Empty) = [[a]]
paths (Node a l r) = (a:) <$> ((paths l)++(paths r))

maskm x y = x ++ drop (length x) y

lv t = foldl maskm [] $ paths t
rv t = foldr (flip maskm) [] $ paths t

-- --------------------------------------

zip' :: [[a]] -> [[a]] -> [[a]]
zip' l [] = l
zip' [] r = r
zip' (x:xs) (y:ys) = (x++y): zip' xs ys

lo :: Btree a -> [[a]]
lo Empty = []
lo (Node a l r) = [[a]] ++ zip' (lo l) (lo r)

lv' t = head <$> lo t
rv' t = last <$> lo t

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