Coursera Scala Functional Programming 1st Week balance brackets

Main.scala

  /**
   * Criteria: Every closing parenthesis ) must have a unique opening ( parenthesis before it
   *
   * For every closing parenthesis, we search for the first opening parenthesis and remove it, and keep going
   */
  def balance(chars: List[Char]): Boolean =
    (chars.indexOf(')'), chars.indexOf('(')) match {
      case (leftMostClosing, leftMostOpening) if leftMostClosing == -1 && leftMostOpening != -1 => false    // no closing / some opening => fail
      case (leftMostClosing, leftMostOpening) if leftMostClosing == -1 && leftMostOpening == -1 => true     // no closing or opening => ok
      case (leftMostClosing, leftMostOpening) if leftMostClosing != 1 && leftMostOpening == -1 => false     // closing but no opening => fail
      case (leftMostClosing, leftMostOpening) if leftMostClosing != -1 && leftMostOpening != -1             // first closing before first opening
        && leftMostClosing < leftMostOpening => false
      case (leftMostClosing, leftMostOpening) => balance(                                                   // first closing after first opening => strip them and recurse
        chars
          .zipWithIndex
          .filter(tuple => tuple._2 != leftMostClosing && tuple._2 != leftMostOpening)
          .map(tuple => tuple._1)
      )
    }

@inakianduaga now go for the "nested is allowed" version :-)