-- linked binary tree with characters in the nodes
data Tree = Empty | Node Char Tree Tree
  deriving (Show, Eq)

-- input as a Tree
input = Node 'a' (Node 'b' (Node 'd' (Node 'g' Empty Empty) (Node 'h' Empty Empty))
                           (Node 'e' Empty Empty))
                 (Node 'c' (Node 'f' Empty Empty) Empty)

-- solution
pivot Empty = Empty
pivot (Node c l r) = go l (Node c Empty r)
  where go Empty rest = rest
        go (Node c l r) rest = go l (Node c r rest)