Inorder traversal of binary tree using stack in kotlin

Kotlin program for Inorder traversal of binary tree using stack. Here problem description and explanation.

// Kotlin program for
// Inorder traversal without recursion
// Using stack

// Binary Tree node
class TreeNode
{
	var data: Int;
	var left: TreeNode ? ;
	var right: TreeNode ? ;
	// Make a tree node
	constructor(data: Int)
	{
		// Set node data
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class StackNode
{
	var element: TreeNode ? ;
	var next: StackNode ? ;
	constructor(element: TreeNode ? )
	{
		this.element = element;
		this.next = null;
	}
}
// Stack Class
class MyStack
{
	var top: StackNode ? ;
	var size: Int;
	constructor()
	{
		this.top = null;
		this.size = 0;
	}
	// Add new element into stack
	fun push(element: TreeNode ? ): Unit
	{
		// Create new node
		val node: StackNode = StackNode(element);
		node.next = this.top;
		// new node is new top
		this.top = node;
		// Increase the size
		this.size += 1;
	}
	// Remove top element of the stack
	fun pop(): Unit
	{
		if (this.top != null)
		{
			// next node is new top
			this.top = this.top?.next;
			// Reduce size
			this.size -= 1;
		}
	}
	// Returns the status of empty or non empty stacks
	fun isEmpty(): Boolean
	{
		if (this.size > 0 && this.top != null)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Return top element of stack
	fun peek(): TreeNode ?
	{
		if (this.isEmpty())
		{
			return null;
		}
		return this.top?.element;
	}
}
class BinaryTree
{
	var root: TreeNode ? ;
	constructor()
	{
		this.root = null;
	}
	// Display Inorder view of binary tree
	fun inorder(): Unit
	{
		if (this.root != null)
		{
			// Make new stack
			val s: MyStack = MyStack();
			var node: TreeNode ? = this.root;
			// Display tree element
			while (!s.isEmpty() || node != null)
			{
				if (node != null)
				{
					// Add current node
					s.push(node);
					// Visit to left subtree
					node = node.left;
				}
				else
				{
					// When node is null
					// Get stack top element
					node = s.peek();
					// Display node value
					print(" " + node!!.data);
					// Remove top element of the stack
					s.pop();
					// Visit to left subtree of current node
					node = node.right;
				}
			}
		}
		else
		{
			print("Empty Linked List\n");
		}
	}
}
fun main(args: Array < String > ): Unit
{
	// Create new tree
	val tree: BinaryTree = BinaryTree();
	/*
	    Construct Binary Tree
	    -----------------------
	        15
	       /  \
	      24   54
	     /    /  \
	    35   62   13
	*/
	// Add tree nodes
	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 result
	tree.inorder();
}

Output

 35 24 15 62 54 13