scala binary tree

binary.scala

/* contravariance required so Empty can be used with Nodes ( Nothing <: T ) */
class Tree[+T]
case object Empty extends Tree[Nothing] 
case class Node[T](val elem: T, val left: Tree[T], val right: Tree[T]) extends Tree[T]

def inOrder[T](t: Tree[T]): List[T] = t match {
  case Empty => Nil
  case Node(e, left, right) => inOrder(left) ::: List(e) ::: inOrder(right)
}

def preOrder[T](t: Tree[T]): List[T] = t match {
  case Empty => Nil
  case Node(e, left, right) => e :: preOrder(left) ::: preOrder(right)
}

def postOrder[T](t: Tree[T]): List[T] = t match {
  case Empty => Nil
  case Node(e, left, right) => postOrder(left) ::: postOrder(right) ::: List(e)
}

def size[T](t: Tree[T]): Int = t match {
  case Empty => 0
  case Node(e, left, right) => 1 + size(left) + size(right)
}

def leafCount[T](t: Tree[T]): Int = t match {
  case Empty => 0
  case Node(e, Empty, Empty) => 1
  case Node(e, left, right) => leafCount(left) + leafCount(right)
}

def leaves[T](t: Tree[T]): List[T] = t match {
  case Empty => Nil
  case Node(e, Empty, Empty) => List(e)
  case Node(e, left, right) => leaves(left) ::: leaves(right)
}

def height[T](t: Tree[T]): Int = t match {
  case Empty => 0
  case Node(e, left, right) => 1 + math.max(height(left), height(right))
}

That's very good, except a misleading error in the comment : "covariance is required", not "contravariance".