Get index of each root on level wise in tree data structure

Question

Hey I am working on tree data structure. I want to know can we get index of each node in level wise. I below diagram represent how I want the value + index. Level A or B represent node value and index value represent index value

                                     Node
                     |                 |                 |
Level A ->           1                 2                 3
index value->        0                 1                 2
                 |       |          |      |          |     |
                 |       |          |      |          |     |
Leve B->         4       5          6      7          8     9
index value->    0       1          2      3          4     5

....// more level
       

How can we achieved index in each level wise. I am adding my logic how I am adding value in each level wise. Could you someone suggest how can I achieve this?

var baseNode: LevelIndex = LevelIndex()
var defaultId = "1234"
fun main() {
    val list = getUnSortedDataListForLevel()
    val tempHashMap: MutableMap<String, LevelIndex> = mutableMapOf()
    list.forEach { levelClass ->
        levelClass.levelA?.let { levelA ->
            val levelOneTempHashMapNode = tempHashMap["level_a${levelA}"]
            if (levelOneTempHashMapNode != null) {
                if (defaultId == levelClass.id && levelOneTempHashMapNode is LevelOne) {
                    levelOneTempHashMapNode.defaultValue = true
                }
                return@let
            }
            val tempNode = LevelOne().apply {
                value = levelA
                if (defaultId == levelClass.id) {
                    defaultValue = true
                }
            }
            baseNode.children.add(tempNode)
            tempHashMap["level_a${levelA}"] = tempNode
        }

        levelClass.levelB?.let { levelB ->
            val levelTwoTempHashMapNode = tempHashMap["level_a${levelClass.levelA}_level_b${levelB}"]
            if (levelTwoTempHashMapNode != null) {
                if (defaultId == levelClass.id && levelOneTempHashMapNode is LevelTwo) {
                    levelTwoTempHashMapNode.defaultValue = true
                }
                return@let
            }
            val tempNode = LevelTwo().apply {
                value = levelB
                if (defaultId == levelClass.id) {
                    defaultValue = true
                }
            }
            val parent =
                tempHashMap["level_a${levelClass.levelA}"] ?: baseNode
            parent.children.add(tempNode)
            tempHashMap["level_a${levelClass.levelA}_level_b${levelB}"] =
                tempNode
        }

        levelClass.levelC?.let { levelC ->
            val tempNode = LevelThree().apply {
                value = levelC
                if (defaultId == levelClass.id) {
                    defaultValue = true
                }
            }
            val parent =
                tempHashMap["level_a${levelClass.levelA}_level_b${levelClass.levelB}"]
                    ?: baseNode
            parent.children.add(tempNode)
        }
    }
}

open class LevelIndex(
    var value: String? = null,
    var children: MutableList<LevelIndex> = arrayListOf()
)

class LevelOne : LevelIndex() {
    var defaultValue: Boolean? = false
}

class LevelTwo : LevelIndex() {
    var defaultValue: Boolean? = false
}

class LevelThree : LevelIndex() {
    var defaultValue: Boolean = false
}

UPDATE

I want index value by root level because, I have one id, I want to match that combination with that id, if that value is present then I am storing that value b true, and need to find that index value.

                                 Node
                     |                 |                 |
Level A ->           1                 2                 3
index value->        0                 1                 2
default value->     false             true             false
                 |       |          |      |          |     |
                 |       |          |      |          |     |
Leve B->         4       5          6      7          8     9
index value->    0       1          2      3          4     5
default value->false   false       true   false     false  false
....// more level

So, Level A I'll get index 1.

For Level B I'll get index 2