inOrder.scala

def inOrder[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,l::ls) => ls.foldLeft(loop(l,output).enqueue(f(v))){case (acc,n) => loop(n,acc)}
    case Tree(v,Nil) => output.enqueue(f(v))
  }
  loop(tree, Queue.empty)
}