Givennpairs of parentheses, write a function to generate all combinations of well-formed parentheses.

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<String>();
        if(n < 0)
            return res;

        helper(res, "", 0, 0, n);
        return res;
    }

    public void helper(List<String> res, String curr, int left, int right, int n) {
        if(curr.length() == 2 * n) {
            if(left == right) {
                res.add(curr);
            }

            return;
        }

        if(right > left)
            return;
        helper(res, curr + "(", left + 1, right, n);
        helper(res, curr + ")", left, right + 1, n);
    }
}