Flatten binary tree in order of postorder traversal in scala

Scala program for Flatten binary tree in order of postorder traversal. Here mentioned other language solution.

/* 
  Scala program for
  Flatten binary tree in order of post-order traversal
*/
// Node of Binary Tree
class TreeNode(var data: Int,
	var left: TreeNode,
		var right: TreeNode)
{
	def this(data: Int)
	{
		// Set node value
		this(data, null, null);
	}
}
class BinaryTree(var root: TreeNode,
	var back: TreeNode)
{
	def this()
	{
		this(null, null);
	}
	// Recursive function
	// Display postorder view of binary tree
	def postOrder(node: TreeNode): Unit = {
		if (node != null)
		{
			postOrder(node.left);
			postOrder(node.right);
			// Print node value
			print("  " + node.data);
		}
	}
	// This are flattening tree nodes in postorder from
	def transform(node: TreeNode): Unit = {
		if (node != null)
		{
			// Recursive executing left and right subtree
			transform(node.left);
			transform(node.right);
			if (this.back == null)
			{
				// This is first node of postorder traversal
				// Get first node of transform tree
				this.root = node;
			}
			else
			{
				// Next node
				this.back.right = node;
				// We taking only one direction
				this.back.left = null;
			}
			this.back = node;
		}
	}
	// This are handling the request of
	// flatten tree nodes in post order from.
	def flattenNode(): Unit = {
		if (this.root == null)
		{
			// When empty tree
			return;
		}
		// Set back node
		this.back = null;
		// Perform flatten operation
		transform(this.root);
		if (this.back != null)
		{
			// Set last node of post order
			this.back.left = null;
			this.back.right = null;
		}
		this.back = null;
	}
	// Display flatten elements of tree
	def showElement(): Unit = {
		if (this.root == null)
		{
			println("\n Empty Tree");
			return;
		}
		println("\n Flatten Tree Node in Postorder : ");
		var temp: TreeNode = this.root;
		// Iterate tree elements
		while (temp != null)
		{
			// Display node value
			print("  " + temp.data);
			// Visit to next node
			temp = temp.right;
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		// New tree
		var tree: BinaryTree = new BinaryTree();
		/*
		    Construct Binary Tree
		    -----------------------
		           1
		          / \ 
		         /   \
		        6     8
		       / \   / \
		      2   3 7   5
		     /   /   \   \
		    9   4    -6   11
		*/
		// Add nodes
		tree.root = new TreeNode(1);
		tree.root.left = new TreeNode(6);
		tree.root.left.left = new TreeNode(2);
		tree.root.right = new TreeNode(8);
		tree.root.right.right = new TreeNode(5);
		tree.root.right.left = new TreeNode(7);
		tree.root.right.left.right = new TreeNode(-6);
		tree.root.left.right = new TreeNode(3);
		tree.root.left.right.left = new TreeNode(4);
		tree.root.left.left.left = new TreeNode(9);
		tree.root.right.right.right = new TreeNode(11);
		// Display tree elements
		println("\n Postorder Nodes : ");
		tree.postOrder(tree.root);
		// Testing
		tree.flattenNode();
		// After transform
		tree.showElement();
	}
}

Output

 Postorder Nodes :
  9  2  4  3  6  -6  7  11  5  8  1
 Flatten Tree Node in Postorder :
  9  2  4  3  6  -6  7  11  5  8  1