Basic Calculator II

class Solution:
    def calculate(self, s: str) -> int:
        i = 0
        cur = prev = res = 0
        cur_oper = "+"
        while i < len(s):
            cur_char = s[i]
            if cur_char.isdigit():
                while i < len(s) and s[i].isdigit():
                    cur = cur * 10 + int(s[i])
                    i += 1
                i -= 1
                if cur_oper == "+":
                    res += cur
                    prev = cur
                elif cur_oper == "-":
                    res -= cur
                    prev = -cur
                elif cur_oper == "*":
                    res -= prev
                    res += prev * cur
                    prev = cur * prev
                else:
                    res -= prev
                    res += int(prev / cur)
                    prev = int(prev / cur)
                cur = 0
            elif cur_char != " ":
                cur_oper = cur_char
            i += 1
        return res