Flatten binary tree in order of postorder traversal in kotlin

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

/* 
  Kotlin 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 ? ;
	constructor(data: Int)
	{
		// Set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinaryTree
{
	var root: TreeNode ? ;
	var back: TreeNode ? ;
	constructor()
	{
		this.root = null;
		this.back = null;
	}
	// Recursive function
	// Display postorder view of binary tree
	fun postOrder(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			this.postOrder(node.left);
			this.postOrder(node.right);
			// Print node value
			print("  " + node.data);
		}
	}
	// This are flattening tree nodes in postorder from
	fun transform(node: TreeNode ? ): Unit
	{
		if (node != null)
		{
			// Recursive executing left and right subtree
			this.transform(node.left);
			this.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.
	fun flattenNode(): Unit
	{
		if (this.root == null)
		{
			// When empty tree
			return;
		}
		// Set back node
		this.back = null;
		// Perform flatten operation
		this.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
	fun 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;
		}
	}
}
fun main(args: Array < String > ): Unit
{
	// New tree
	val tree: BinaryTree = BinaryTree();
	/*
	    Construct Binary Tree
	    -----------------------
	           1
	          / \ 
	         /   \
	        6     8
	       / \   / \
	      2   3 7   5
	     /   /   \   \
	    9   4    -6   11
	*/
	// Add nodes
	tree.root = TreeNode(1);
	tree.root?.left = TreeNode(6);
	tree.root?.left?.left = TreeNode(2);
	tree.root?.right = TreeNode(8);
	tree.root?.right?.right = TreeNode(5);
	tree.root?.right?.left = TreeNode(7);
	tree.root?.right?.left?.right = TreeNode(-6);
	tree.root?.left?.right = TreeNode(3);
	tree.root?.left?.right?.left = TreeNode(4);
	tree.root?.left?.left?.left = TreeNode(9);
	tree.root?.right?.right?.right = 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