Sum of all the grandchild nodes of binary tree in kotlin

Kotlin program for Sum of all the grandchild nodes of binary tree. Here mentioned other language solution.

/*
    Kotlin program for
    Sum of nodes in binary tree whose grandparents exists
*/
// 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;
	}
	fun preOrder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Print node value
			print("  " + node.data);
			// Visit left subtree
			this.preOrder(node.left);
			// Visit right subtree
			this.preOrder(node.right);
		}
	}
	fun grandchildSum(node: TreeNode ? , level : Int): Int
	{
		if (node != null)
		{
			var result: Int = 0;
			if (level > 2)
			{
				// Get the node
				result = node.data;
			}
			// Visit left subtree and right subtree
			return result + 
              this.grandchildSum(node.left, level + 1) + 
              this.grandchildSum(node.right, level + 1);
		}
		else
		{
			return 0;
		}
	}
}
fun main(args: Array < String > ): Unit
{
	// Create new binary trees
	val tree: BinaryTree = BinaryTree();
	/*
	         4   
	        /  \                          
	       /    \    
	      12     7    
	     / \      \               
	    2   3      1
	       / \    / 
	      6   8  5
	     /        
	    9                        
	  ----------------
	  Constructing binary tree    
	*/
	// Add nodes
	tree.root = TreeNode(4);
	tree.root?.left = TreeNode(12);
	tree.root?.left?.right = TreeNode(3);
	tree.root?.left?.right?.left = TreeNode(6);
	tree.root?.left?.right?.left?.left = TreeNode(9);
	tree.root?.left?.right?.right = TreeNode(8);
	tree.root?.left?.left = TreeNode(2);
	tree.root?.right = TreeNode(7);
	tree.root?.right?.right = TreeNode(1);
	tree.root?.right?.right?.left = TreeNode(5);
	// Display tree node
	print("\n Tree Nodes ");
	tree.preOrder(tree.root);
	// Find node sum
	// 2 + 3 + 6 + 9 + 8 + 1 + 5 = 34
	val result: Int = tree.grandchildSum(tree.root, 1);
	// Display result
	println("\n Result : " + result);
}

Output

 Tree Nodes   4  12  2  3  6  9  8  7  1  5
 Result : 34