trait Tree
case class Node(i: Int, left: Tree, right: Tree) extends Tree
case class Leaf(i:Int) extends Tree

val tree = Node(4, Leaf(1), Node(7, Leaf(3), Leaf(8)))

def mirror(node: Tree): Tree = {
  node match {
    case n: Node => Node(n.i, mirror(n.right), mirror(n.left))
    case l: Leaf => Leaf(l.i)
  }
}

mirror(tree)