Hands-On Design Patterns with Swift

What is an reverse polish notation calculator?

Reverse Polish Notation (RPN), and it is also known as postfix notation. In RPN, the operators follow their operands, and mathematical operations such as 3 + 4 or 4 * 3 + 2 are written as 3 4 + and 4 3 * 2 +. At first glance, it might look counterintuitive, but it's really handy once you get used to it. One useful advantage of an RPN calculator is that it's easier to solve nested operations with it. It's easier because it doesn't need to use parentheses to resolve the logical priority between operations. For example:

3 * ( 2 + 5) must be written as 3 2 5 + *

The mechanic is based on the concept of stacking: each operand is added to a stack. When an operator is inserted, like add, multiply,