22 Generate Parentheses

使用helper 函数帮助进行迭代，类似于二叉树遍历中后序遍历，先处理左右子树，最后处理根节点。helper 函数的三个参数，剩余的open, 剩余的close，以及当前的字串。

class Solution(object):
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        """
        ## 
        def helper(left, right, current):
            if left == 0 and right == 0:
                return current
            if left > right:
                return current
            if current == []:
                return helper(left-1, right, ["("])
            ans = []
            for i in current:
                if left > 0:
                    ans.extend(helper(left-1, right, [ i+"(" ]))
                if right > 0 and left < right: 
                    ans.extend(helper(left, right-1, [ i+")" ]))
            return ans
        return helper(n,n,[])