Generate Parentheses

 1 class Solution {
 2 public:
 3     vector<string> generateParenthesis(int n) {
 4         vector<string> result;
 5         if(n <= 0) return result;
 6         
 7         helper(result, "", n, n);
 8         return result;
 9     }
10     void helper(vector<string>& result, string temp, int left, int right){
11         if(left > right) return;
12         
13         if(left == 0 && right == 0){
14             result.push_back(temp);
15             return;
16         }
17         
18         if(left > 0)
19             helper(result, temp + "(", left - 1, right);
20         
21         if(right > 0)
22             helper(result, temp + ")", left, right - 1);
23         
24     }
25 };