Insert items to a binary tree in kotlin

TreeSort.kt

package com.emma.recursion

import kotlin.test.assertEquals

/**
 * Insert items to a binary tree
 *
 * The side effect is, if the tree is printed with an inorder traversal, the items are sorted in asc order
 *
 * Worst complexity: n * log(n) (balanced)
 * Average complexity: n * log(n)
 * Best complexity: n * log(n)
 * Space complexity: n
 * 
 */
class Node(private val value: Int) {
    private var leftNode: Node? = null
    private var rightNode: Node? = null

    fun addNode(newNode: Node) {
        when {
            newNode.value >= value -> {
                when (rightNode) {
                    null -> rightNode = newNode
                    else -> rightNode!!.addNode(newNode)
                }
            }
            newNode.value < value -> {
                when (leftNode) {
                    null -> leftNode = newNode
                    else -> leftNode!!.addNode(newNode)
                }
            }
        }
    }

    override fun toString(): String {
        var response = ""

        if (leftNode != null) response += "$leftNode, "

        response += value

        if (rightNode != null) response += ", $rightNode"

        return response
    }
}


fun main() {
    val itemsToSort = arrayOf(5, 8, 1, 3, 2, 4, 7, -1)
    val tree = Node(itemsToSort[0])
    for (item in 1..itemsToSort.lastIndex) {
        tree.addNode(Node(itemsToSort[item]))
    }
    println(tree)
    assertEquals(tree.toString(), "-1, 1, 2, 3, 4, 5, 7, 8", "bug in the sorting algorithmn :-(")
}