[LeetCode] Generate Parentheses

Well, there are two ways to add a open or close parenthesis to the current string.

If number of ( is less than n, you can add (;
If number of ) is less than number of (, you can add ).

Maintain a res for all the possible parenthesis and a temporary string sol for the current answer. Now we have the following code.

 1 class Solution {
 2 public:
 3     vector<string> generateParenthesis(int n) {
 4         vector<string> res;
 5         string sol;
 6         genParen(sol, 0, 0, n, res);
 7         return res;
 8     }
 9 private:
10     void genParen(string& sol, int open, int close, int total, vector<string>& res) {
11         if (open == total && close == total) {
12             res.push_back(sol);
13             return;
14         }
15         if (open < total) {
16             sol += '(';
17             genParen(sol, open + 1, close, total, res);
18             sol.resize(sol.length() - 1);
19         }
20         if (close < open) {
21             sol += ')';
22             genParen(sol, open, close + 1, total, res);
23             sol.resize(sol.length() - 1);
24         }
25     }
26 };