[rybosome]

Here's the tail-call optimized version, won't blow the stack. Technically idiomatic, but way too cryptic to be serious code. Clearly I need more outlets for writing Scala. =)

    @tailrec
    def getValuesFromNode(node: Node, accValues: List.empty[Int], nodeList: List.empty[Node]): List[Int] = node match {
      case Fork(left, right) => getValuesFromList(accValues, nodeList ++ List(left, right))
      case Leaf(value) => getValuesFromList(accValues ++ List(value), nodeList)
    }

    @tailrec
    def getValuesFromList(accValues: List.empty[Int], nodeList: List.empty[Node]): List[Int] = nodeList match {
      case x :: xs => getValuesFromNode(x, accValues, xs)
      case _ => accValues
    }