Leetcode 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: "((()))", "(()())", "(())()", "()(())", "()()()"

Solve the problem on leetcode

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        if(n<=0)   return res;
        //string tmp;
        dfs(res,"",n,n);
        return res;
        
    }
    void dfs(vector<string> &res,string tmp,int left,int right)
    {
        if(left==0&&right==0)
        {
            res.push_back(tmp);
            return;
        }
        if(left>0) dfs(res,tmp+'(',left-1,right);
        if(left<right)  dfs(res,tmp+')',left,right-1); 
    }
};