Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
For example, given n = 3, a solution set is:
[ "((()))", "(()())", "(())()", "()(())", "()()()" ]
解题思路
从左到右生成相应的符号序列，首先添加‘（’，左右括号成对出现且个数一致，右边括号计数大于左边括号时需要添加‘）’，直到计数都为0时表示当前生成一个有效的序列，依次递归操作
class Solution(object):
def generateParenthesis(self, n):
"""
:type n: int
:rtype: List[str]
"""
def gernerate_str(sstr,lnum,rnum,result=[]):
if lnum==0 and rnum==0:
result.append(sstr)
return
if lnum>0:
gernerate_str(sstr+'(',lnum-1,rnum,result)
if rnum>lnum:
gernerate_str(sstr+')',lnum,rnum-1,result)
result=[]
gernerate_str('',n,n,result)
return result
