from typing import Any, List """ The Reverse Polish Nation also known as Polish postfix notation or simply postfix notation. https://en.wikipedia.org/wiki/Reverse_Polish_notation Classic examples of simple stack implementations Valid operators are +, -, *, /. Each operand may be an integer or another expression. """ def evaluate_postfix(postfix_notation: list) -> int: """ >>> evaluate_postfix(["2", "1", "+", "3", "*"]) 9 >>> evaluate_postfix(["4", "13", "5", "/", "+"]) 6 >>> evaluate_postfix([]) 0 """ if not postfix_notation: return 0 operations = {"+", "-", "*", "/"} stack: List[Any] = [] for token in postfix_notation: if token in operations: b, a = stack.pop(), stack.pop() if token == "+": stack.append(a + b) elif token == "-": stack.append(a - b) elif token == "*": stack.append(a * b) else: if a * b < 0 and a % b != 0: stack.append(a // b + 1) else: stack.append(a // b) else: stack.append(int(token)) return stack.pop() if __name__ == "__main__": import doctest doctest.testmod()