在Scala中编写树递归的规范方法是什么？

sealed trait Tree[+A]
case object Leaf extends Tree[Nothing]
case class Node[A](value: A, left: Tree[A], right: Tree[A]) extends Tree[A]

def sumTree(tree: Tree[Int]): Int = tree match {
  case Leaf => 0
  case Node(value, left, right) => value + sumTree(left) + sumTree(right)
}

def countNodes(tree: Tree[Any]): Int = tree match {
  case Leaf => 0
  case Node(_, left, right) => 1 + countNodes(left) + countNodes(right)
}

def findMax(tree: Tree[Int]): Int = tree match {
  case Leaf => Int.MinValue
  case Node(value, left, right) => value.max(findMax(left)).max(findMax(right))
}