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:
[ "((()))", "(()())", "(())()", "()(())", "()()()" ]
class Solution:def generateParenthesis(self, n):""":type n: int:rtype: List[str]"""res = []def gen(s="", open=0, close=0):if len(s) is 2 * n:res.append(s)else:if open < n:gen(s + "(", open + 1, close)if close < open:gen(s + ")", open, close + 1)gen()return res
