224. Basic Calculator

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 .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:

You may assume that the given expression is always valid.
Do not use the eval built-in library function.

class Solution:
    def calculate(self,s):
        """
        :type s: str
        :rtype: int
        """
        num,sign,i = 0,1,0
        op = []
        while i <len(s):
            if s[i].isdigit():
                start = i
                while i<len(s) and s[i].isdigit():
                    i += 1
                n = int(s[start:i])
                num += n * sign
                continue
            if s[i]==‘+‘:
                sign = 1
                i += 1
                continue
            if s[i]==‘-‘:
                sign = -1
                i += 1
                continue
            if s[i]==‘(‘:
                op.append(num)
                op.append(sign)
                num = 0
                sign = 1
                i += 1
                continue
            if s[i]==‘)‘:
                num = num * op.pop()
                num += op.pop()
                i += 1
                continue
            i += 1
        return num