A solver using Scala's flatmap to solve the Johnny Hates Math problem found at http://www.spoj.com/problems/ANARC07J/ (NB: this is to find all solutions to the problem, not to satisfy the SPOJ IO specification)

SolverUsingFlatMap.scala

package jhm

case class SolverUsingFlatMap( digits: String, total: Int, maxDigitsPerNumber: Int = 5
                        ) extends JHMSolver(digits, total, maxDigitsPerNumber) {

  override def solve(digits: String, total: Int, maxDigitsPerNumber: Int = 5): List[List[Int]] = {
    
    def getSolutionsForNextNumber(leftCount: Int): List[List[Int]] = {
      val (leftStr, rightStr) = digits.splitAt(leftCount)
      if (leftCount > 1 && leftStr(0) == '0') List.empty // Disallow numbers starting with zero
      else {
        val left: Int = leftStr.toInt
        val remTotal: Int = total - left
        if (remTotal < 0) List.empty  // Exceeded target
        else {
          if (rightStr == "") {
            if (remTotal == 0) List(List(left)) // Reached target exactly
            else List.empty // Digits ran out before reaching target
          }
          else {
            // Recursively solve the smaller problem:
            val subNumbers = solve(rightStr, remTotal)
            subNumbers map (left :: _)
          }
        }
      }
    }

    val solutions = (
      (1 to math.min(maxDigitsPerNumber, digits.length)).toList
      flatMap getSolutionsForNextNumber
    )
    solutions
  }
}