basic calculator

class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        res = 0
        num = 0
        sign = 1   # current sign (1 means '+', -1 means '-')

        for ch in s:
            if ch.isdigit():
                num = num * 10 + int(ch)
            elif ch in "+-":
                res += sign * num
                num = 0
                sign = 1 if ch == "+" else -1
            elif ch == "(":
                # push current result and sign
                stack.append(res)
                stack.append(sign)
                res = 0
                sign = 1
            elif ch == ")":
                res += sign * num
                num = 0
                res *= stack.pop()  # sign before parenthesis
                res += stack.pop()  # previous result
            # ignore spaces

        return res + sign * num