A recursive way to increment a sequence of Integers - which are in fact Digits [0 ... 9].

sequence-increment.scala

/** Given a Sequence of Integers Seq(2, 3, 4) - which represents the number 234 base 10 - increments
  * by one without converting types.
  *
  * @param s Seq[Int] The original sequence representation of the number
  * @return The incremented by one Sequence
  */
def sequenceIncrement(s: Seq[Int]): Seq[Int] = {
  sequenceIncrementRec(s.reverse, Seq.empty, carry = true).reverse
}

/** Recursively increment a sequence of integers
  *
  * @param seq Seq[Int] A sequence of integers (e.g. Seq(2,7,1) => 2 * 10^0 + 7 * 10^1 + 1 * 10^2)
  * @param accumulator Seq[Int] The sequence to carry on during recursion
  * @param carry Boolean true if balance carried forward
  * @return The incremented by one sequence where the unit is the head element (e.g. Seq(3,7,1))
  */
def sequenceIncrementRec(seq: Seq[Int], accumulator: Seq[Int], carry: Boolean): Seq[Int] = seq match {
  case Nil => if(carry) accumulator :+ 1 else accumulator
  case head::tail =>
    val current = head + (if(carry) 1 else 0)
    val curried = current % 10
    sequenceIncrementRec(tail, accumulator :+ curried, curried != current)
}