def isValid(self, s):

        if not s:
            return True

        dict = {'(':')', '[':']', '{':'}'}
        list = []

        if s[0] in dict.values():
            return False

        for i in range(len(s)):
            if s[i] in dict.keys():
                list.append(s[i])
            if s[i] in dict.values() and len(list) == 0:
                return False
            if s[i] in dict.values() and dict[list.pop()] != s[i]:
                return False

        if len(list):
            return False

        return True