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:

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

Code:

class Solution {
public:
    void generate(int l, int r, string s, vector<string> &res){
        if(l==0&&r==0){
           res.push_back(s);
           return;
        }    
        if(l>0)
            generate(l-1,r,s+'(',res);
        if(r>l)
            generate(l,r-1,s+')',res);
    }
    
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        string s;
        if(n==0) return res;
        generate(n,n,s,res);
        return res;
    }
};