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