3211 生成不含相邻零的二进制字符串

class Solution {
public:
    vector<string> res;
    void find(string s,int l){
        if(s.size()==l){
            res.push_back(s);
        }
        else if(s[s.size()-1]=='1'){
            find(s+"1",l);
            find(s+"0",l);
        }
        else{
            find(s+"1",l);
        }
    }
    vector<string> validStrings(int n) {
        int l=n;
        if(l==1){
            return {"1","0"};
        }
        if(l==2){
            return {"10","01","11"};
        }
        find("0",l);
        find("1",l);
        return res;
    }
};

重点掌握如何输出