Insertion in binary search tree using recursion in kotlin

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

// Kotlin program for
// Insertion in binary search tree by using recursion
class TreeNode
{
	var data: Int;
	var left: TreeNode ? ;
	var right: TreeNode ? ;
	constructor(data: Int)
	{
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinarySearchTree
{
	var root: TreeNode ? ;
	constructor()
	{
		this.root = null;
	}
	// Insert a node element
	fun 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 = this.addNode(node.left, data);
			}
			else
			{
				// When new element is higher to current node
				node.right = this.addNode(node.right, data);
			}
			// important to manage root node
			return node;
		}
		else
		{
			return TreeNode(data);
		}
	}
	// Display preorder
	fun preorder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Display node value
			print("  " + node.data);
			// Visit to left subtree
			this.preorder(node.left);
			// Visit to right subtree
			this.preorder(node.right);
		}
	}
	fun inorder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Visit to left subtree
			this.inorder(node.left);
			// Display node value
			print("  " + node.data);
			// Visit to right subtree
			this.inorder(node.right);
		}
	}
	fun postorder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Visit to left subtree
			this.postorder(node.left);
			// Visit to right subtree
			this.postorder(node.right);
			// Display node value
			print("  " + node.data);
		}
	}
}
fun main(args: Array < String > ): Unit
{
	val tree: BinarySearchTree = 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