checkParentheses.scala

object Solution extends App {
    val expression = readLine.trim

    val asso = Map(')' -> '(', ']' -> '[', '}' -> '{')
    val isop = asso.values.toSet
    val iscl = asso.keys.toSet

    def cleanStack(c:Char)(implicit stack: List[Char]) =
        stack.filter(_ == c).tail ++ stack.filterNot(_ == c)

    def isBalanced(expr:List[Char])(implicit opened: List[Char] = Nil): Boolean =
        expr match {
            case o :: t if isop(o) => isBalanced(t)(expr.head::opened)
            case c :: t if iscl(c) => if (!opened.contains(asso(c))) false else isBalanced(t)(cleanStack(asso(c)))
            case h :: t            => isBalanced(t)
            case Nil               => opened.isEmpty
        }

    println( isBalanced(expression.toList) )
}