Leetcode generate-parentheses(生成括号， DFS）

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        string out;
        DFS(n,0,0,out,res);
        return res;
    }
    void DFS(int n,int l,int r,string &out,vector<string> &res)
    {
        if(l==n&&r==n)
        {
            res.push_back(out);
            return;
        }
        else
        {
            if(l<n)
            {
                out.push_back(‘(‘);
                DFS(n,l+1,r,out,res);
                out.pop_back();
            }
            if(l>r)
            {
                out.push_back(‘)‘);
                DFS(n,l,r+1,out,res);
                out.pop_back();
            }
        }
    }
};