856. Score of Parentheses——string

class Solution(object):
    def repeatedNTimes(self, S):
        num = 0
        tmp = S[0]
        a = []
        for i in range(len(S)):
            if S[i] == "(":
                tmp = "("
                a.append("(")
            elif tmp == "(" and S[i] == ")":
                num += 2**(len(a)-1)
                tmp = ")"
                a.pop()
            elif tmp == ")" and S[i] == ")":
                a.pop()
        return num