[leetCode]Generate Parentheses

思路：每次插入一对括号，注意对于之前出现过的字符串就不要在处理了，否则会TLE.

 1 #include <string>
 2 #include <vector>
 3 #include <set>
 4 using namespace std;
 5 class Solution {
 6 public:
 7     set<string> strset; 
 8     vector<string> generateParenthesis(int n) {
 9         string s = "";
10         vector<string> ret;
11         if(n <= 0) return ret;
12         generate(n,s,0,ret);
13         return ret;
14     }
15     void generate(int n, string s, int index, vector<string> &ret){
16         s.insert(index,"()");
17         if(strset.find(s) != strset.end()) return;// get TLE without this sentence;
18         strset.insert(s);
19         if(n <= 1){
20             ret.push_back(s);    
21         }else{
22             for(int i = 0; i < s.length(); i++){
23                 generate(n-1,s,i,ret);
24             }
25         }
26     }
27 };