[LC] 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(object):
    def calculate(self, s):
        """
        :type s: str
        :rtype: int
        """
        sign = 1
        res = 0
        stack = []
        index = 0
        while index < len(s):
            char = s[index]
            if char.isdigit():
                num = int(char)
                while index + 1 < len(s) and s[index + 1].isdigit():
                    num = 10 * num + int(s[index + 1])
                    index += 1
                res += sign * num
            elif char == '+':
                sign = 1
            elif char == '-':
                sign = -1
            elif char == '(':
                stack.append(res)
                stack.append(sign)
                res = 0
                sign = 1
            elif char == ')':
                res = stack.pop() * res + stack.pop()
            index += 1
        return res