Leetcode: Basic Calculator

Question

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:
“1 + 1” = 2
” 2-1 + 2 ” = 3
“(1+(4+5+2)-3)+(6+8)” = 23
Note: Do not use the eval built-in library function.

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Solution

Get idea from here, here2

class Solution(object):    def calculate(self, s):        """        :type s: str        :rtype: int        """        if s=='':            return 0        res, num, sign = 0, 0 , 1        stack = []        for elem in s:            if elem.isdigit():                num = num*10 + ord(elem) - ord('0')            else:                if elem == '+':                    res += num*sign                    sign = 1                elif elem == '-':                    res += num*sign                    sign = -1                elif elem=='(':                    stack.append(res)                    stack.append(sign)                    res, sign = 0, 1     # calculate a new string, like '0+expression'                elif elem==')':                    res += num*sign                    res *= stack.pop(-1)                    res += stack.pop(-1)                    sign = 1                else:                    continue                num = 0        res += num*sign        return res