LeetCode224：基本计算器（栈）

 解题思路:

1、双栈模拟，一个用来存数，一个用来存操作符。需要考虑 '('后面紧跟'+'、'-'这种情况

2、递归：遇到左括号开始递归，遇到右括号结束递归，返回值。

class Solution:
    def calculate(self, s):
        s = '('+s+')'
        stack = []
        op = []
        val = 0
        flag = False
        for i in range(len(s)):
            if ord('0')<=ord(s[i])<=ord('9'):
                val=val*10+ord(s[i])-ord('0')
                flag= True
            elif s[i]=='(':
                op.append('(')
                stack.append('(')
            elif s[i]==')':
                if flag:
                    stack.append(val)
                    flag = False
                    val = 0
                ans = 0
                while op[-1]!='(' and stack[-1]!='(':
                    if op[-1]=='+':
                        ans = ans + stack[-1]
                    elif op[-1]=='-':
                        ans = ans - stack[-1]
                    _,_= stack.pop(),op.pop()

                if op[-1]=='(' and stack[-1]=='(':
                    _ = op.pop(),stack.pop()
                elif op[-1]=='(':
                    ans+=stack[-1]
                    _ = stack.pop()
                    _ = op.pop(), stack.pop()
                else:
                    print('error')
                    exit()
                stack.append(ans)
            elif s[i]=='+':
                if flag:
                    stack.append(val)
                    flag=False
                    val = 0
                op.append('+')
            elif s[i]=='-':
                if flag:
                    stack.append(val)
                    flag =False
                    val=0
                op.append('-')
            elif s[i]==' ':
                continue
        return stack[-1]
s = Solution().calculate("0+0+(-0+0)")
print(s)