Find kth smallest element in BST kotlin

Kotlin program for Find kth smallest element in BST. Here more solutions.

// Kotlin program for
// K’th smallest element in Binary Search Tree
class TreeNode
{
	var data: Int;
	var left: TreeNode ? ;
	var right: TreeNode ? ;
	constructor(data: Int)
	{
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinarySearchTree
{
	var root: TreeNode ? ;
	var counter: Int;
	var element: TreeNode ? ;
	constructor()
	{
		this.root = null;
      	this.counter = 0;
      	this.element = null;
	}
	// Insert a node element
	fun insert(node: TreeNode ? , data : Int): TreeNode ?
	{
		if (node != null)
		{
			if (node.data >= data)
			{
				// When new element is smaller or equal to current node
				node.left = this.insert(node.left, data);
			}
			else
			{
				// When new element is higher to current node
				node.right = this.insert(node.right, data);
			}
			// Important to manage new node
			return node;
		}
		else
		{
			return TreeNode(data);
		}
	}
	// Find the kth smallest node in BST
	fun findKthSmallest(node: TreeNode ? , k : Int): Unit
	{
		if (node != null)
		{
			// Visit left subtree
			this.findKthSmallest(node.left, k);
			if (this.counter == k)
			{
				// Stop recursion
				return;
			}
			if (this.element == null || 
                this.element!!.data < node.data)
			{
				// Modified counter variable by one
				this.counter += 1;
				// Collect possible result
				this.element = node;
			}
			// Visit right subtree
			this.findKthSmallest(node.right, k);
		}
	}
	fun printKthSsmallest(k: Int): Unit
	{
		if (k <= 0)
		{
			// When given k is not valid positive values
			println("Invalid node position " + k);
		}
		else if (this.root == null)
		{
			// When BST are no have elements
			println("Empty Binary Search Tree");
		}
		else
		{
			// Reset values
			this.counter = 0;
			this.element = null;
			// Find result
			this.findKthSmallest(this.root, k);
			if (this.counter < k)
			{
				// If In case given kth smallest node are
				// Not exist in binary search tree, then
				println("Given " + k + 
                        " smallest node are not exist " + this.counter);
			}
			else
			{
				println("[" + k + 
                        "] smallest node : " + this.element!!.data);
			}
		}
	}
	// Handle request to add new node
	fun addNode(key: Int): Unit
	{
		this.root = this.insert(this.root, key);
	}
}
fun main(args: Array < String > ): Unit
{
	val tree: BinarySearchTree = BinarySearchTree();
	/*
	    Make Tree
	    ----------
	        7
	       / \ 
	      /   \
	     4     9
	    / \   / \
	   3   5 8   10
	      /    
	     4     
	*/
	// Add node
	tree.addNode(7);
	tree.addNode(4);
	tree.addNode(3);
	tree.addNode(5);
	tree.addNode(9);
	tree.addNode(8);
	tree.addNode(10);
	tree.addNode(4);
	// Testing
	tree.printKthSsmallest(6);
	tree.printKthSsmallest(4);
	tree.printKthSsmallest(3);
}

Output

[6] smallest node : 9
[4] smallest node : 7
[3] smallest node : 5