Leetcode 22 Generate Parentheses

<pre name="code" class="python">class Solution:
    # @param {integer} n
    # @return {string[]}
    def generateParenthesis(self, n):
        
        def Recursive(left,right,string,res):
            if left==0 and right==0:
                 res.append(string)
                 return
            if left>right: return
            if left>0:
                Recursive(left-1,right,string+'(',res)
            if right>0:
                Recursive(left,right-1,string+')',res)
            
        res=[]
        Recursive(n,n,'',res)
        return res