solution.scala

def findLettersOrDigits(chars: Array[Char], i: Int, f: Char => Boolean): String = {
  if (i == chars.length) return ""

  val char = chars(i)

  if (f(char)) {
    char.toString + findLettersOrDigits(chars, i + 1, f)
  }
  else {
    ""
  }
}

def findCloseBracket(chars: Array[Char], i: Int, count: Int): Int = {
  if (count == 0) return i - 1

  if (chars(i) == '[') {
    findCloseBracket(chars, i + 1, count + 1)
  }
  else if (chars(i) == ']') {
    findCloseBracket(chars, i + 1, count - 1)
  }
  else {
    findCloseBracket(chars, i + 1, count)
  }
}

def decodeString(s: String): String = {
  def decode(chars: Array[Char], i: Int): String = {
    if (i >= chars.length) return ""

    val char = chars(i)

    if (char.isLetter) {
      val letters = findLettersOrDigits(chars, i, _.isLetter)
      letters + decode(chars, i + letters.length)
    }
    else {
      val repeat = findLettersOrDigits(chars, i, _.isDigit)
      val last = findCloseBracket(chars, i + repeat.length + 1, 1)
      val repeated = decodeString(s.substring(i + repeat.length + 1, last)) * repeat.toInt
      repeated + decode(chars, last + 1)
    }
  }

  decode(s.toCharArray, 0)
}