Kotlin, memory saving - kotlin

Question

I'm working on Splay Tree. I have a lot of command input (add, delete and etc), Hence the output is also huge (32MB of text in a txt file)
I have a memory problem, I am currently using two MultitableList and I have 200 MB of virtual memory (128 MB is needed for the project)
I run on Linux with the command:
$ kotlinc Kotlin.kt -include-runtime -d outPutFile.jar
$ /usr/bin/time -v java -jar outPutFile.jar
result: Maximum resident set size (kbytes): 249732 (With each launch different sizes but about 200)
how can i reduce the size? I need to change the size after each cycle
class SplayTree {
var ROOT: Node? = null
class Node(val key: Long, var value: String?) {
var _parent: SplayTree.Node? = null
var _RightChild: SplayTree.Node? = null
var _LeftChild: SplayTree.Node? = null
... there is a code here ...
// for output
override fun toString(): String {
if (_parent == null) {
return "[${key} ${value}]"
} else {
return "[${key} ${value} ${_parent!!.key}]"
}
}
}
... there is a code here ...
override fun toString(): String {
if (ROOT == null) {
return "_"
}
println(ROOT)
var NOWqueueList: List<Node?> = listOf(ROOT)
var BABYqueue: MutableList<Node?> = MutableList(0) { null }
//var NOWqueueList = Array<Node?>(1, {ROOT}) // Array
//var BABYqueue = Array<Node?>(1, {null}) // Array
//var n = 1 // Array
for (h in 1 until height(ROOT)) {
// for Array
//var pos = -1
//n *= 2
//BABYqueue = Array<Node?>(n, {null}) //for future line
for (ROOTList in NOWqueueList) {
//pos++ // Array
if ( ROOTList != null){
//left
if (ROOTList.haveLeftBaby()) {
//BABYqueue[pos] = ROOTList._LeftChild // Array
BABYqueue.add(ROOTList._LeftChild)
print(ROOTList._LeftChild.toString())
print(" ")
} else {
//BABYqueue[pos] = null // Array
BABYqueue.add(null)
print("_ ")
}
//pos++ // Array
//right
if (ROOTList.haveRightBaby()) {
//BABYqueue[pos] = ROOTList._RightChild // Array
BABYqueue.add(ROOTList._RightChild)
print(ROOTList._RightChild.toString())
print(" ")
} else {
//BABYqueue[pos] = null
BABYqueue.add(null)
print("_ ")
}
} else{ //если пустой то + 2 "_"
//BABYqueue[pos] = null // Array
//pos++ // Array
//BABYqueue[pos] = null // Array
BABYqueue.add(null)
BABYqueue.add(null)
print("_ _ ")
}
}
//NOWqueueList.clear() //worked when was MultitableList
NOWqueueList = BABYqueue.toList() // equate
//NOWqueueList = BABYqueue.clone() // Array
//println(NOWqueueList.joinToString(" ")) // вывожу готовый
println()
BABYqueue.clear()
}
//NOWqueueList.clear()
BABYqueue.clear()
return " end="
}
}
fun main() {
... there is a code here ...
}
I tried using Array, it still came out as with MultitableList