535. Encode and Decode TinyURL

https://leetcode.com/problems/encode-and-decode-tinyurl/description/
class Solution {
    unordered_map<string, string> encodeMap;
    unordered_map<string, string> decodeMap;
    string dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
public:

    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        if(encodeMap.count(longUrl)) return encodeMap[longUrl];
        string shortUrl = "http://tinyurl.com/";
        int n = encodeMap.size(), nd = dict.size();
        while(n) {
            shortUrl += dict[n%nd];
            n /= nd;
        }
        encodeMap[longUrl] = shortUrl;
        decodeMap[shortUrl] = longUrl;
        return shortUrl;
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        return decodeMap[shortUrl];
    }
};