Scala solution (as a worksheet) to the string decoding problem: https://leetcode.com/problems/decode-string

StringDecode.sc

def splitWhile[A](it: Seq[A])(pred: A => Boolean): (Seq[A], Seq[A]) = {
  (it.takeWhile(pred), it.dropWhile(pred))
}

def splitParenContentsAndRest(s: Seq[Char]): (Seq[Char], Seq[Char]) = {
  assert(s.head == '[')

  val parenCounts = s.scanLeft(0) {
    case (parenCount, '[') => parenCount + 1
    case (parenCount, ']') => parenCount - 1
    case (parenCount, _) => parenCount
  }

  val parenEnd = parenCounts.tail.indexOf(0)
  val (paren, rest) = s.splitAt(parenEnd + 1)
  val unpackedParen = paren.slice(1, paren.length - 1)

  (unpackedParen, rest)
}

def decode(s: Seq[Char]): String = {
  if (s.isEmpty) {
    ""
  } else {
    val (digits, paren) = splitWhile(s)(_.isDigit)
    if (digits.nonEmpty) {
      val (parenContents, rest) = splitParenContentsAndRest(paren)
      val decodedParen = decode(parenContents)
      val count = digits.mkString("").toInt
      val decodedRepeated = List.fill(count)(decodedParen)
      decodedRepeated.mkString("") + decode(rest)
    } else {
      val (letters, rest) = splitWhile(s)(_.isLetter)
      if (letters.nonEmpty) {
        letters.mkString("") + decode(rest)
      } else {
        sys.error("Unexpected format: " + s)
      }
    }
  }
}

decode("3[a]2[bc]")
decode("3[a2[c]]")
decode("2[abc]3[cd]ef")
decode("100[leetcode]")