给定一个括号数量n，生成所有可能的括号组合字符串

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
__title__ = ''
__author__ = 'thinkreed'
__mtime__ = '2017/3/24'

"""


class Solution(object):
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        idea from https://discuss.leetcode.com/topic/17510/4-7-lines-python/2
        """

        def generate(p, left, right, parens=[]):
            if left:         generate(p + '(', left - 1, right)
            if right > left: generate(p + ')', left, right - 1)
            if not right:    parens += p,
            return parens

        return generate('', n, n)