def isInterleave_TLE(self, s1, s2, s3):
        """
        dfs
        Time Limit Exceeded
        :param s1:
        :param s2:
        :param s3:
        :return: boolean
        """
        if not s3:
            return True
        letter = s3[0]
        if s1 and s1[0] == letter:
            if self.isInterleave(s1[1:], s2, s3[1:]):
                return True
        if s2 and s2[0] == letter:
            if self.isInterleave(s1, s2[1:], s3[1:]):
                return True
        return False