[leetcode] 22. 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) {
        int left = n;
        int right = n;
        vector<string> res;
        c(res, string(""), left, right);
        return res;
    }
    
    void c(vector<string>& res, string s, int left, int right){
        if(left == 0 && right == 0) res.push_back(s);
        if(left > 0) c(res, s+'(', left-1,right);
        if(right > left) c(res, s+')', left, right-1);
    }
};