Functional Programming Principles in Scala - Exercise 2: Parentheses Balancing

ParenthesesBalancing.sc

/**
    * Exercise 2
    */
  def balance(chars: List[Char]): Boolean = {
    def difference(acc: Int, ch: List[Char]): Int = {
      val head = ch.head
      val tail = ch.tail
      val isLeft = head == '('
      val isRight = head == ')'
      val notEmptyTail = tail.nonEmpty
      if (acc == 1) -1
      else if (!isLeft && !isRight)
        if (notEmptyTail) difference(acc, tail) else acc
      else if (isLeft)
        if (notEmptyTail) difference(acc - 1, tail) else acc - 1
      else if (notEmptyTail) difference(acc + 1, tail) else acc + 1
    }

    difference(0, chars) == 0
  }

This exercise is copied from the Functional Programming Principles in Scala course, week 1. Find more on my website.