有效的括號

思路：用一個字符串來模擬入棧和出棧，如果是左括號則入棧，如果是右括號則出棧，如果是一個有效的括號，那麼最終這個棧一定是空的

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if not s:
            return True
        # if s.startswith(")") or s.startswith("]") or s.startswith("}"):
        #     return False
        #用一個字符串來模擬入棧出棧
        s1 = ""
        for ss in s:
            if ss == "(" or ss == "[" or ss == "{":
                s1 += ss
                #print s1
            else:
                if len(s1) < 1:
                    return False
                if ss == ")":
                    if s1[-1] == "(":
                        s1 = s1[:-1]
                        continue
                    else:
                        return False
                elif ss == "]":
                    if s1[-1] == "[":
                        s1 = s1[:-1]
                        continue
                    else:
                        return False
                elif ss == "}":
                    if s1[-1] == "{":
                        s1 = s1[:-1]
                        continue
                    else:
                        return False
        return s1 == ""