535. Encode and Decode TinyURL

class Solution {
public:
    Solution(){
        dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        short2long.clear();
        long2short.clear();
        srand(time(NULL));
    }    
    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        if(long2short.count(longUrl))
            return "http://tinyurl.com/" + long2short[longUrl];
        string randStr;
        for(int i = 0; i < 6; i++){
            randStr.push_back(dict[rand() % 62]);
        }
        
        int idx = 0;
        while(short2long.count(randStr)){
            idx %= 6;
            randStr[idx] = dict[rand() % 62];
        }
        short2long[randStr] = longUrl;
        long2short[longUrl] = randStr;
        return "http://tinyurl.com/" + randStr;
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        string randStr = shortUrl.substr(shortUrl.find_last_of("/")+1);
        return short2long.count(randStr) ? short2long[randStr] : shortUrl;
    }
private:
    unordered_map<string, string> short2long, long2short;
    string dict;
};

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