Inorder traversal of binary tree with recursion in kotlin

Kotlin program for Inorder traversal of binary tree with recursion. Here problem description and explanation.

/* 
  Kotlin Program for
  inorder tree traversal of a Binary Tree
  using recursion
*/
// Binary Tree Node
class TreeNode
{
	var data: Int;
	var left: TreeNode ? ;
	var right: TreeNode ? ;
	constructor(data: Int)
	{
		// Set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinaryTree
{
	var root: TreeNode ? ;
	constructor()
	{
		this.root = null;
	}
	// Display Inorder view of binary tree
	fun inorder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Visit left subtree
			this.inorder(node.left);
			//Print node value
			print("  " + node.data);
			// Visit right subtree
			this.inorder(node.right);
		}
	}
}
fun main(args: Array < String > ): Unit
{
	// Create new tree
	val tree: BinaryTree = BinaryTree();
	/*
	    Make A Binary Tree
	    ----------------
	        15
	       /  \
	      24   54
	     /    /  \
	    35   62   13
	*/
	// Add tree TreeNode
	tree.root = TreeNode(15);
	tree.root?.left = TreeNode(24);
	tree.root?.right = TreeNode(54);
	tree.root?.right?.right = TreeNode(13);
	tree.root?.right?.left = TreeNode(62);
	tree.root?.left?.left = TreeNode(35);
	// Display Tree Node
	tree.inorder(tree.root);
}

Output

  35  24  15  62  54  13