code.rb

  def evaluate(expression)
    stack = []

    expression.split.each do |word|
      case word
      when "+"
        stack << (stack.pop + stack.pop)
      when "-"
        arg1 = stack.pop
        arg2 = stack.pop
        stack << (arg2 - arg1)
      when "*"
        stack << (stack.pop * stack.pop)
      when "/"
        arg1 = stack.pop
        arg2 = stack.pop
        stack << (arg2 / arg1)
      else
        stack << Float(word)
      end
    end
    stack.last
  end