The Reverse Polish Notation

1) if there's a number, you stack it on the top

2) if there's an operating sign, you do what it says with the top 2 numbers from the stack, e.g. if the top number is 2, there's a 7 right under it, and you have a "^", you raise 7 to the 2nd power and get 49 - the result replaces the numbers used to create it - in this example, the stack of [5, 12, 8, 7, 2] becomes [5, 12, 8, 49]