Leetcode#22||Generate Parentheses

public class Solution {    public List<String> generateParenthesis(int n) {        List<String> result = new ArrayList<String>();                if (n <= 0) {            return result;        }                dfs(result, "", n, n);                return result;    }        private void dfs(List<String> result, String s, int left, int right) {        // prune        if (left > right) {            return;        }        // find solution        if (left == 0 && right == 0) {            result.add(s);            return;        }                if (left > 0) {            dfs(result, s + "(", left - 1, right);        }                if (right > 0) {            dfs(result, s + ")", left, right - 1);        }    }}