题目描述：
Given a string s representing an expression, implement a basic calculator to evaluate it.

Example 1:
Input: s = “1 + 1”
Output: 2

Example 2:
Input: s = " 2-1 + 2 "
Output: 3

Example 3:
Input: s = “(1+(4+5+2)-3)+(6+8)”
Output: 23

Constraints:
1 <= s.length <= 3 * 105
s consists of digits, ‘+’, ‘-’, ‘(’, ‘)’, and ’ '.
s represents a valid expression.
来源：力扣（LeetCode）

这道题官方分类为困难题，其实这个题主要就是栈的用法：

# -*- coding: utf-8 -*-
"""
Created on Wed Mar 10 11:16:50 2021
题目要求实现一个简单的计算器，
输入一个字符串
输出计算的结果
字符串中包括数字，+，-，（，），空格
@author: Chauncy
"""
import typing

class Solution:
    def calculate(self, s: str) -> int:
        n = len(s)
        cur_num = 0
        res = 0
        tmp_stack = []
        sign = 1
        for i in range(n):
            if s[i].isdigit():
                cur_num = 10 * cur_num + int(s[i])
            elif s[i] == '+':
                res += sign * cur_num
                cur_num = 0
                sign = 1
            elif s[i] == '-':
                res += sign * cur_num
                cur_num = 0
                sign = -1
            elif s[i] == '(':
                tmp_stack.append(res)
                tmp_stack.append(sign)
                sign = 1
                res = 0
            elif s[i] == ')':
                res += sign * cur_num
                cur_num = 0
                res *= tmp_stack.pop()
                res += tmp_stack.pop()
                
        if cur_num != 0:
            res += sign * cur_num
        return res
sol = Solution()

s = "(1+(4+5+2)-3)+(6+8)"
print(sol.calculate(s))