Reverse Polish Notation Calculator

gistfile1.rb

Create an RPNCalculator class which can evaluate expressions
written in Reverse Polish notation.

It should have an evaluate instance method which takes as its 
input a valid RPN expression and returns its evaluation. Your 
calculator only needs to handle addition, multiplication, and 
subtraction (not division).

Operators and numbers should be separated by a single space.

For example,

calc = RPNCalculator.new

calc.evaluate('1 2 +')   # => 3
calc.evaluate('2 5 *')   # => 10
calc.evaluate('50 20 -') # => 30

# The general rule is that 'A B op' is the same as 'A op B'
# i.e., 5 4 - is 5 - 4.
calc.evaluate('70 10 4 + 5 * -') # => 0



class RPNCalculator

  def evaluate(expression)

    operators = ["+", "-", "*"]
    stack = []
    array = expression.split(" ")

      array.each do |i|
        if operators.include?(i)
          second_operand = stack.pop
          first_operand = stack.pop

          stack.push(first_operand.send(i, second_operand))
        else
          stack.push(i.to_i)
        end
      end

    stack.pop
  end
end