Generate Parentheses

来源：Leetcode

原帖：http://oj.leetcode.com/problems/generate-parentheses/

题目：
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:
"((()))", "(()())", "(())()", "()(())", "()()()"

代码：

 class Solution {  
 public:  
   vector<string> generateParenthesis(int n) {  
     vector<string> result;  
     generateParenthesisHelper(n, n, "", result);  
     return result;  
   }  
   
   void generateParenthesisHelper(int left, int right, string s, vector<string> &result) {  
     if (left == 0 && right == 0) {  
       result.push_back(s);        
     }  
     if (left > 0) {  
       generateParenthesisHelper(left - 1, right, s + "(", result);        
     }  
     if (right > left) {  
       generateParenthesisHelper(left, right - 1, s + ")", result);        
     }  
   }  
 };