Sample LinkedList implementation supporting flatten()

MyLinkedList.kt

import java.lang.IllegalStateException

class MyLinkedList : Iterable<Any> {
  var head: Node = Node(Any())

  fun add(value: Any) {
    var n: Node = head
    while (n.next != null) {
      n = n.next!!
    }
    n.next = Node(value)
  }

  fun flatten() {
    fun insert(node: Node, value: Any): Node {
      val nn = node.next
      val newNode = Node(value)
      newNode.next = nn
      node.next = newNode
      return newNode
    }

    var n: Node = head

    while (n.next != null) {
      val nn = n.next!!
      if (nn.value is MyLinkedList) {
        n.next = nn.next // remove the sublist to flatten from the list
        nn.value.forEach { n = insert(n, it) }
      }
      n = nn
    }
  }

  override fun iterator(): Iterator<Any> {
    return object : Iterator<Any> {
      private var ptr: Node = head
      override fun hasNext() = ptr.next != null

      override fun next(): Any {
        if (ptr.next == null) throw IllegalStateException(
            "hasNext() should be called to verify the next value exists before next()")
        val ret = ptr.next!!.value
        ptr = ptr.next!!
        return ret
      }
    }
  }

  override fun toString(): String {
    val sb = StringBuilder("[")
    this.forEach { sb.append(it).append(", ") }
    if (sb.length > 1) {
      sb.setLength(sb.length - 2)
    }
    sb.append("]")
    return sb.toString()
  }
}

class Node(val value: Any) {
  var next: Node? = null
}

fun main(args: Array<String>) {
  val list1 = MyLinkedList()
  list1.add(1)
  list1.add(2)
  val list2 = MyLinkedList()
  list2.add(3)
  list2.add(list1)
  list2.add(4)

  list2.flatten()
  println(list2) // [3, 1, 2, 4]
}