leetcode 22. 括号生成 dfs

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string>ret;
        if(n==0)
            return ret;
        string s;
        int l=0,r=0;
        dfs(ret,n,s,l,r);
        return ret;
    }
    void dfs(vector<string>&ret,int len,string s,int l,int r)
    {
        if(r==len){
            ret.push_back(s);
            return;
        }
        else
        {
            if(l<len)
            dfs(ret,len,s+"(",l+1,r);
            if(r<l)
            dfs(ret,len,s+")",l,r+1);
            return;
        }
    }
};