RPN.rb

class RPNCalculator
  def initialize(expression)
    @expression = expression 
  end 
  
  def evaluate(expression) 
    container = @expression.split(' ')
    num_container = []
    container.each do |x| 
      if !(x == '+' || x == '-' || x == '*')
        num_container << x
      elsif x == '+'
        add = num_container[-2].to_i + num_container[-1].to_i
        2.times do 
          num_container.pop
        end
        num_container << add
      elsif x == '-'
        dif = num_container[-2].to_i - num_container[-1].to_i
        2.times do 
          num_container.pop
        end
        num_container << dif 
      elsif x == '*'
        mult = num_container[-2].to_i * num_container[-1].to_i
        2.times do 
          num_container.pop
        end
        num_container << mult
      end
    end
    
    num_container
 end
end