問５回答 40分くらい

Test.scala

// http://www.softantenna.com/wp/software/5-programming-problems/
// Problem 5
case class Expr(str: String, ret: Int) {
  def plus(n: Int) = Expr(str + " + " + n, ret + n)
  def minus(n: Int) = Expr(str + " - " + n, ret - n)
  def concat(n: Int) = {
    val next = last match {
      case 0 => ret * 10 + n
      case x if (x < 0) => ret - x + (x * 10) - n
      case x if (x > 0) => ret - x + (x * 10) + n
    }
    Expr(str + n.toString, next)
  }

  def last: Int = str.split(" ").toList.takeRight(2) match {
    case n :: Nil => 0
    case "+" :: n :: Nil => n.toInt
    case "-" :: n :: Nil => 0 - n.toInt
    case _ => throw new IllegalStateException()
  }

  def calc(n: Int) = plus(n) :: minus(n) :: concat(n) :: Nil

  override def toString = str + " = " + ret
}

object Test {
  def main(args: Array[String]): Unit = {
    val list = (2 to 9).toList
    val seed = List(Expr("1", 1))
    list.foldLeft(seed) { case (ret, n) =>
      ret.flatMap(_.calc(n))
    }.filter(_.ret == 100).foreach(println)
  }
}