224. Basic Calculator

用了一个栈，当遇到（时， 把当前的结果存起来，以及括号前面的符号存起来，result置为零，重新开始算，遇到)把result * 前面存下来的符号 加上（前面的result

class Solution(object):
    def calculate(self, s):
        """
        :type s: str
        :rtype: int
        """
        if len(s) == 0:
            return 0
        sign = 1
        result = 0
        i = 0
        stack = []
        while i < len(s):
            if s[i] == ' ':
                i += 1
            elif s[i] >= '0' and s[i] <= '9':
                sum = int(s[i])
                while i + 1 < len(s) and s[i+1] >= '0' and s[i+1] <= '9':
                    i += 1
                    sum = sum * 10 + int(s[i])
                result += sum * sign
                i += 1
            elif s[i] == '+':
                sign = 1
                i += 1
            elif s[i] == '-':
                sign = -1
                i += 1
            elif s[i] == '(':
                stack.append(result)
                stack.append(sign)
                result = 0
                sign = 1
                i += 1
            elif s[i] == ')':
                result = result * stack.pop()
                result += stack.pop()
                i += 1
        return result