224. Basic Calculator

這道題在無數(shù)次修修補補的過程中也算是AC了，自己的答案實在是太冗長，就不貼在這了。找了個簡潔一些的答案。

class Solution(object):
    def calculate(self, s):
        res, num, sign, stack = 0, 0, 1, []
        for ss in s:
            if ss.isdigit():
                num = 10*num + int(ss)
            elif ss in ["-", "+"]:
                res += sign*num
                num = 0
                sign = [-1, 1][ss=="+"]
            elif ss == "(":
                stack.append(res)
                stack.append(sign)
                sign, res = 1, 0
            elif ss == ")":
                res += sign*num
                res *= stack.pop()
                res += stack.pop()
                num = 0
        return res + num*sign