题目：
题解：
class Solution:def calculate(self, s: str) -> int:n = len(s)stack = []preSign = '+'num = 0for i in range(n):if s[i] != ' ' and s[i].isdigit():num = num * 10 + ord(s[i]) - ord('0')if i == n - 1 or s[i] in '+-*/':if preSign == '+':stack.append(num)elif preSign == '-':stack.append(-num)elif preSign == '*':stack.append(stack.pop() * num)else:stack.append(int(stack.pop() / num))preSign = s[i]num = 0return sum(stack)
