postOrder.scala

def postOrder[T,S](tree: Tree[T], f: T => S): Queue[S] = {
  def loop(g: Tree[T], output: Queue[S]): Queue[S] = g match {
    case Tree(v,rest) => rest.foldLeft(output){case (agg,node) => loop(node,agg)}.enqueue(f(v))
  }
  loop(tree,Queue.empty)
}