leetcode 22. Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

 class Solution {
	 void do_once(set<string>&candi)
	 {
		 set<string>newcandi; set<string>hist;
		 for (set<string>::iterator it = candi.begin(); it != candi.end(); it++)
		 {
			 string str = *it;
			 for (int j = 0; j <= str.length(); j++)
			 {
				 string s = str;
				 s.insert(s.begin() + j, '(');
				 if (hist.find(s) == hist.end())
				 {
					 hist.insert(s);
					 for (int h = j + 1; h <= s.length(); h++)
					 {
						 string ss = s;
						 ss.insert(ss.begin() + h, ')');
						 if (newcandi.find(ss) == newcandi.end())
							 newcandi.insert(ss);
					 }
				 }
			 }
		 }
		 candi = newcandi;
	 }
 public:
	 vector<string> generateParenthesis(int n) {
		 vector<string>re;
		 if (n == 0)
			 return re;
		 set<string>candi;
		 candi.insert("()");
		 while (n > 1)
		 {
			 do_once(candi);
			 n--;
		 }
		 return vector<string>(candi.begin(), candi.end());
	 }
 };

accepted