Leetcode 227 — Facebook Interview Question

class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        num = 0
        prev_op = '+'

        for index, char in enumerate(s):
            if char.isdigit():
                num = num * 10 + int(char)
            if index == len(s)-1 or char in ['+', '-', '*', '/']:
                if prev_op == '+':
                    stack.append(num)
                if prev_op == '-':
                    stack.append(-num)
                if prev_op == '*':
                    prev_num = stack.pop()
                    num = prev_num * num
                    stack.append(num)
                if prev_op == '/':
                    prev_num = stack.pop()
                    num = int(prev_num / num)
                    stack.append(num)
                num = 0
                prev_op = char
        return sum(stack)