Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> result = new ArrayList<String>();
        helper(n, n, "", result);
        return result;
    }

    public void helper(int left, int right, String path, List<String> result) {
        if (left == 0 && right == 0) {
            result.add(path);
            return;
        }
        if (left > right || left < 0 || right < 0) {
            //when the number of "(" left to add is larger than the number of ")", stop
            return;
        }
        helper(left - 1, right, path + "(", result);
        helper(left, right - 1, path + ")", result);
    }
}

class Solution(object):
    def generateParenthesis(self, n):
        """
        :type n: int
        :rtype: List[str]
        """

        if not n:
            return []

        ret = []
        self.helper(n, n, "", ret)
        return ret

    def helper(self, left, right, curr, ret):
        if left == 0 and right == 0:
            ret.append(curr)
            return
        if left > right or left < 0 or right < 0:
            return
        self.helper(left - 1, right, curr + "(", ret)
        self.helper(left, right - 1, curr + ")", ret)