22.Generate Parentheses (String; dfs)

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        if(n == 0) return ret;
        
        len = n*2;
        dfsLeft(n, 0, "");
        return ret;
    }
    
    void dfsRight(int lNum, int rNum, string str){//add right parenthese
        while(rNum){ 
            str += ‘)‘;
            dfsLeft(lNum, --rNum, str);
        }
        if(str.length() == len){
            ret.push_back(str);
        }
    }
    
    void dfsLeft(int lNum, int rNum, string str){//add left parenthese
        while(lNum){
            str += ‘(‘;
            dfsRight(--lNum, ++rNum, str);
        }
    }
private:
    int len;
    vector<string> ret;

};