lintcode

class Solution {
public:
    /*
     * @param n: n pairs
     * @return: All combinations of well-formed parentheses
     */
    vector<string> generateParenthesis(int n) {
        // write your code here
        vector<string> ans;
        dfs(0, 0, "", ans, n);
        return ans;
    }
    void dfs(int left, int right, string res, vector<string> &ans, int n){
        if(left < right || (left > n || right > n)) return ;
        if(left == n && right == n){
                ans.push_back(res);
                return ;
        }
        string tmp = res;
        string tmp1 = res;
        tmp += ')';
        tmp1 += '(';
        dfs(left, right + 1, tmp, ans, n);
        dfs(left + 1, right, tmp1, ans, n);

    }
};

右边的括号不能比左边多