535 Encode and Decode TinyUrl

class Solution {
public:
    string dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    //每次更新两个map
    map<string,string>L_TO_S;//key=longUrl,value=shortUrl
    map<string,string>S_TO_L;//key=shortUrl,value=longUrl
    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        if(L_TO_S.find(longUrl)!=L_TO_S.end())return L_TO_S[longUrl];//防止一个longUrl产生多个shortUrl,浪费空间
        string s ="";
        while(s.size()<6){//随机产生一个六位字符串
            srand(time(NULL));
            s=s+dict[rand()%62];
        }
        L_TO_S[longUrl]=s;
        S_TO_L[s]=longUrl;
        return s;
        
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        if(S_TO_L.find(shortUrl)!=S_TO_L.end())return S_TO_L[shortUrl];
        string l ="";
        while(l.size()<6){
            srand(time(NULL));
            l=+dict[rand()%62];
        }
        L_TO_S[shortUrl]=shortUrl+l;
        S_TO_L[shortUrl+l]=shortUrl;
        return l;
    }
};

// Your Solution object will be instantiated and called as such:
// Solution solution;
// solution.decode(solution.encode(url));