parseArray.kt

import java.util.Stack

fun parse(s: String): MutableList<Any> {
  val stack = Stack<MutableList<Any>>()
  stack.add(mutableListOf())
  var i = 0
  while (i < s.length) {
    val ch = s[i]
    when (ch) {
      '[' -> stack.push(mutableListOf())
      ']' -> stack.pop().also { stack.peek().add(it) }
      else ->
        if (ch in '0'..'9') {
          var j = i
          while (s[j] in '0'..'9') j++
          stack.peek().add(s.substring(i until j).toInt())
          i = j - 1
        }
    }
    i++
  }
  return stack.pop()
}

fun main(args: Array<String>) {
  val list = parse("[1, 2, [3], [4, [5, 6], []]]")
  println(list)
}