Balanced parentheses example.

Parens.scala

object Parens extends App {
  
  def balanced (input: String): Boolean = {
    val stack = collection.mutable.Stack.empty[String]
    import stack._
    for (i <- input) i match {
      case '(' => push("(")
      case '[' => push("[")
      case ')' => if (isEmpty) return false else if (pop() != "(") return false
      case ']' => if (isEmpty) return false else if (pop() != "[") return false
    } // for
    true
  } // balanced

  def balanced2 (input: String): Boolean = {
    if (input.length % 2 != 0) return false
    var (p, ps, b, bs) = (0, 0, 0, 0)
    for (i <- input) i match {
      case '(' => { p +=1; ps = 0; if (bs > 0) bs += 1 }
      case '[' => { b +=1; bs = 0; if (ps > 0) ps += 1 }
      case ')' => { p -=1; ps += 1 + (if (bs > 0) bs else 0) }
      case ']' => { b -=1; bs += 1 + (if (ps > 0) ps else 0) }
    } // for
    if ((p == 0) && (b == 0) && (bs + ps <= input.length)) true else false
  } // balanced2

  List("([][])",
       "([()])",
       "((()))",
       "([)]()",
       "())())") foreach {
	 e => {
	   println(e)
	   if (balanced2(e)) println(" is balanced!") else println(" is not balanced")
	 }
       }

} // Parens