construct a balanced binary tree from a sorted array.

toTree.hs

{-
    construct a balanced binary tree from a _sorted_ array.
    from haskell course exercise.
-}

data BinaryTree = Branch Integer BinaryTree BinaryTree
                | Leaf
                deriving (Show, Ord, Eq)

insert :: Integer -> BinaryTree -> BinaryTree
insert i Leaf = Branch i Leaf Leaf
insert i (Branch v l r) 
  | i < v = Branch v (insert i l) r
  | otherwise = Branch v l (insert i r)

c :: [Integer] -> BinaryTree
c = foldl (flip insert) Leaf . b

b :: [Integer] -> [Integer]
b [] = []
b xs = let p = m xs in p : b (takeWhile (< p) xs) ++ b (dropWhile (<= p) xs)

m :: [Integer] -> Integer
m [x] = x
m [_,x] = x
m xs = m $ tail $ init xs