Scala training Binary Tree method

BinTree.scala

sealed abstract class Tree
case class Branch(value: Int, left: Tree, right: Tree) extends Tree
case object Empty extends Tree

object TreeObject {
  def max(tree: Tree): Int = tree match {
    case Branch(v, l, r) => math.max(math.max(v, this.max(l)), this.max(r))
    case Empty => 0
  }

  def min(tree: Tree): Int = tree match {
    case Branch(v, l, r) => math.min(math.min(v, this.min(l)), this.min(r))
    case Empty => Int.MaxValue
  }

  def depth(tree: Tree): Int = tree match {
    case Branch(v, l, r) => 1 + math.max(depth(l), depth(r))
    case Empty => 0
  }
}