Use scala to exchange left/right children of a binary tree

Node.scala

package handson

class Node(l: Node, r: Node, n: String) 
{
  var left = l
  var right = r
  var name = n
}

Program.scala

package handson

object Program 
{
  def printTree(root: Node)
  {
    print(root.name)
    print(" ")
    if(root.left != null) printTree(root.left)
    if(root.right != null) printTree(root.right)
  }
  
  def dotTree(root: Node)
  {
    println("digraph tree {")
    dotTreeRecur(root)
    println("}")
  }
  
  def dotTreeRecur(root: Node)
  {
    if(root.left != null)
    {
    	println("\t" + root.name + " -> " + root.left.name)
    	dotTreeRecur(root.left)
    }
    	
    if(root.right != null)
    {
    	println("\t" + root.name + " -> " + root.right.name)
    	dotTreeRecur(root.right)
    }
  }
  
  def exchange(root: Node)
  {
    val tmp = root.left
    root.left = root.right
    root.right = tmp
    
    if(root.left != null) exchange(root.left)
    if(root.right != null) exchange(root.right)
    
  }
  
  def main(args: Array[String]): Unit = 
  {
    // construct the tree
    val nd = new Node(null, null, "D")
    val ne = new Node(null, null, "E")
    val nf = new Node(null, null, "F")
    val ng = new Node(null, null, "G")
    
    val nb = new Node(nd, ne, "B")
    val nc = new Node(nf, ng, "C")
    val na = new Node(nb, nc, "A")
    
    // print - exchange - print
    printTree(na)
    println()
    
    dotTree(na);
    println()
    
    exchange(na)
    
    printTree(na)
    println()
    
    dotTree(na);
    println()
    
  }

}