gistfile1.scala

  val codeToChars: Map[Char, String] =
    Map(
      '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
      '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ"
    )

  /**
   * Inverse of codeToChars,
   * e.g. 'A' -> '2', 'B' -> '2'.
   */
  val charCode: Map[Char, Char] =
    for {
      (digit, chars) <- codeToChars     
      char <- chars                         
    } yield char -> digit