Encode and Decode TinyURL

class Solution {
private:
    string s;
    unordered_map<string, string> HashMap;
public:
    Solution() {
        HashMap = {};
        for (int i = 0 ; i < 26 ; ++i) {
            if (i < 10) s += ('0' + i);
            s += string(1, 'a' + i) + string(1, 'A' + i);
        }
    }
    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        string ans;
        for (int i = 0 ; i < 6 ; ++i) {
            ans += s[rand() % s.size()];
        }
        HashMap[ans] = longUrl;
        return ans;
    }
    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        return HashMap[shortUrl];
    }
};