First attempt to rewrite some of the Solution codebase to swift.

validInput-bad.swift

func validInput(input:String) -> Bool {
  
  var last = ""
  if let x = equation.last { last = x }
  
  switch last {
  case "+", "-", "*", "(", "":
    switch input {
    case let x where input.toInt() != nil:
      return true
    case "(":
      return true
    default:
      return false
    }
  case let x where last.toInt() != nil:
    switch input {
    case "+", "-", "*", ")" where parenthesesStack > 0:
      return true
    default:
      return false
    }
  case ")":
    switch input {
    case "+", "-", "*", "(", ")":
      return true
    default:
      return false
    }
  default:
    return false
  }
}