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class RPNCalculator

  def initialize
  end

  def evaluate(string)
    input = string.split(' ')
    while (input.size > 1)
      op_ind = input.index {|e| e =~ /[+*-]/ }

      if op_ind.nil?
        return input
      else
        operator = input.delete_at(op_ind)
        first_arg = input[op_ind - 2].to_i
        second_arg = input[op_ind - 1].to_i
        output = first_arg.send(operator, second_arg)
        
        input[op_ind - 2] = output.to_s
        input.delete_at(op_ind - 1)
      end
    end
    input
  end
end

calc = RPNCalculator.new
puts calc.evaluate('5 1 2 + 4 * + 3 -')