Reverse Polish Notation Kata

The reverse polish notation allow to write mathematical expressions without the need for parenthesis.

A RPN expression contains either

• A number
• A sequence E1 E2 OP where E1 and E2 are expressions and OP an operator (hence the name, operator comes after the operands)

To evaluate a RPN expression, we use the following algorithm:

• Split the expression in small tokens
• Create a list of values (called a "stack")
• For each token:
  • If the token is a number, push it to the stack
  • If the token is an operator, pop two values from the stack and push the result to the stack

Examples:

20 2 +
[20], [20, 2], [22]

4 2 + 3 -
[4], [4,2], [6], [6, 3], [3]

Write the code that can evaluate an expression (as a string) and return a result as an int.

Follow the TDD rules as close as possible and try to make the smallest possible cycles.

Notes:

• You can assume the expression is well formed : only natural numbers, all tokens separated by space
• You can assume that the "stack" only contain one element at the end

• Implement different arities, like sqrt:

9 sqrt
3

or max:

12 10 14 9 max
14