Basic RPN implmementation

rpn.py

#
# Postfix Notation Calculator
#
# Postfix notation, is a way of writing mathematical
# expressions where each operator is preceded by the
# two operands it applies to and looks something
# like 2 5 3 + * instead of (5 + 3 ) * 2
#
# Write a function that takes a postfix notation expression
# returns the result of evaluating the expression.
#
# Implement the following operators [+, -, *]
#
# eg.
# "3 11 +" -> 14
# "3 11 5 + -" -> -13
# "2 3 11 + 5 - *" -> 18

import collections
import operator

OPERATORS = {
    "+": operator.add,
    "-": operator.sub,
    "*": operator.mul,
}


def resolve(string: str) -> int:
    tokens = string.split(" ")
    stack = collections.deque()

    for token in tokens:
        if token in OPERATORS:
            op1, op2 = stack.pop(), stack.pop()
            stack.append(OPERATORS[token](op1, op2))
        else:
            stack.append(int(token))

    return stack.pop()


tests = []
tests.append("3 11 +")
tests.append("3 11 5 + -")
tests.append("2 3 11 + 5 - *")

for test in tests:
    print(resolve(test))