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:

"((()))", "(()())", "(())()", "()(())", "()()()"

Java

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> rst = new ArrayList<String>();
        if(n < 1) {
            return rst;
        }
        helper(rst, "", n, n);
        return rst;
    }
    private void helper(List<String> rst, String s, int left, int right) {
        if(left < 0 || right < 0 || left > right) {
            return;
        }
        if(left == 0 && right == 0) {
            rst.add(s);
            return;
        }
        helper(rst, s + "(", left - 1, right);
        helper(rst, s + ")", left, right - 1);
    }
}