radixSort.scala

def radixSort(arr: Seq[Int]): Seq[Int] = {
  def fn(state: Seq[Int], d: Int): Seq[Int] = {
    val ten_power_d = Iterator.fill(d)(10).product
    val buckets = state.groupBy(_ / ten_power_d % 10).toMap
    buckets.keys.toSeq.sorted.foldLeft(Seq.empty[Int]) { case (acc, i) =>
      acc ++ buckets(i)
    }
  }

  val digits = Iterator.iterate(arr) {
    _.map(_ / 10)
  }.takeWhile {
    _.exists(_ > 0)
  }.size

  0.to(digits).foldLeft(arr)(fn)
}