leetcode-Generate Parentheses

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(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