Inorder traversal of binary tree using stack in scala

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

// Scala 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
	def this(data: Int)
	{
		// Set node data
		this(data, null, null);
	}
}
class StackNode(var element: TreeNode,
	var next: StackNode)
{
	def this(element: TreeNode)
	{
		this(element, null);
	}
}
// Stack Class
class MyStack(var top: StackNode,
	var size: Int)
{
	def this()
	{
		this(null, 0);
	}
	// Add new element into stack
	def push(element: TreeNode): Unit = {
		// Create new node
		var node: StackNode = new 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
	def 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
	def isEmpty(): Boolean = {
		if (this.size > 0 && this.top != null)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Return top element of stack
	def peek(): TreeNode = {
		if (isEmpty())
		{
			return null;
		}
		return this.top.element;
	}
}
class BinaryTree(var root: TreeNode)
{
	def this()
	{
		this(null);
	}
	// Display Inorder view of binary tree
	def inorder(): Unit = {
		if (this.root != null)
		{
			// Make new stack
			var s: MyStack = new 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");
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		// Create new tree
		var tree: BinaryTree = new BinaryTree();
		/*
		    Construct Binary Tree
		    -----------------------
		        15
		       /  \
		      24   54
		     /    /  \
		    35   62   13
		*/
		// Add tree nodes
		tree.root = new TreeNode(15);
		tree.root.left = new TreeNode(24);
		tree.root.right = new TreeNode(54);
		tree.root.right.right = new TreeNode(13);
		tree.root.right.left = new TreeNode(62);
		tree.root.left.left = new TreeNode(35);
		// Display result
		tree.inorder();
	}
}

Output

 35 24 15 62 54 13