Insertion in binary search tree using recursion in scala

Scala program for Insertion in binary search tree using recursion. Here problem description and explanation.

// Scala program for
// Insertion in binary search tree by using recursion
class TreeNode(var data: Int,
	var left: TreeNode,
		var right: TreeNode)
{
	def this(data: Int)
	{
		this(data, null, null);
	}
}
class BinarySearchTree(var root: TreeNode)
{
	def this()
	{
		this(null);
	}
	// Insert a node element
	def addNode(node: TreeNode, data: Int): TreeNode = {
		if (node != null)
		{
			if (node.data >= data)
			{
				// When new element is smaller or
				// equal to current node
				node.left = addNode(node.left, data);
			}
			else
			{
				// When new element is higher to current node
				node.right = addNode(node.right, data);
			}
			// important to manage root node
			return node;
		}
		else
		{
			return new TreeNode(data);
		}
	}
	// Display preorder
	def preorder(node: TreeNode): Unit = {
		if (node != null)
		{
			// Display node value
			print("  " + node.data);
			// Visit to left subtree
			preorder(node.left);
			// Visit to right subtree
			preorder(node.right);
		}
	}
	def inorder(node: TreeNode): Unit = {
		if (node != null)
		{
			// Visit to left subtree
			inorder(node.left);
			// Display node value
			print("  " + node.data);
			// Visit to right subtree
			inorder(node.right);
		}
	}
	def postorder(node: TreeNode): Unit = {
		if (node != null)
		{
			// Visit to left subtree
			postorder(node.left);
			// Visit to right subtree
			postorder(node.right);
			// Display node value
			print("  " + node.data);
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		var tree: BinarySearchTree = new BinarySearchTree();
		/*
		         10
		        / \
		       /   \
		      4     15
		     / \   /
		    3   5 12
		    -------------
		    Build binary search tree
		*/
		tree.root = tree.addNode(tree.root, 10);
		tree.addNode(tree.root, 4);
		tree.addNode(tree.root, 3);
		tree.addNode(tree.root, 5);
		tree.addNode(tree.root, 15);
		tree.addNode(tree.root, 12);
		// Display tree nodes
		println("Preorder ");
		tree.preorder(tree.root);
		println("\nInorder ");
		tree.inorder(tree.root);
		println("\nPostorder ");
		tree.postorder(tree.root);
	}
}

Output

Preorder
  10  4  3  5  15  12
Inorder
  3  4  5  10  12  15
Postorder
  3  5  4  12  15  10