How to create a node tree from a node list using kotlin

list_tree_list.kt

data class Node(
  val id: String,
  val name: String,
  val parentId: String? = null,
  val childs: MutableList<Node> = mutableListOf()
)

// replace with:
//data class Node(
//  val id: String,
//  val name: String,
//  val parentId: String? = null,
//  val childs: List<Node>? = null)

/**
 * +-1
 * | +-3
 * | +-4
 * |   +-5
 * +-2
 *   +-6
 */
fun main() {
  val init = listOf(
    Node("1", "Node 1"),
    Node("2", "Node 2"),
    Node("3", "Node 3", "1"),
    Node("4", "Node 4", "1"),
    Node("5", "Node 5", "4"),
    Node("6", "Node 6", "2"),
  )
  val tree = listToTree(init)
  val list = treeToList(tree)
  if (list.size == init.size) {
    println("You've done it right!")
  } else {
    println("Nope, try it again!")
  }
}

fun listToTree(list: List<Node>): List<Node> {
  val mapTmp = list.map { it.id to it }.toMap().toMutableMap()
  list.filter { it.parentId != null && mapTmp[it.parentId] != null }
    .forEach { current ->
      mapTmp[current.id] = current
      mapTmp[current.parentId]?.childs?.add(current)
    }
  return mapTmp.values.filter { it.parentId == null }
}

fun treeToList(tree: List<Node>): List<Node> {
  val flattenList = mutableListOf<Node>()
  tree.forEach {
    flattenList.add(Node(it.id, it.name, it.parentId))
    it.childs.let { list ->
      flattenList.addAll(treeToList(list))
    }
  }
  return flattenList
}