535.cpp

class Solution {
public:
    Solution() {
        for(string::size_type i = 0; i < chars.size(); i++) {
            charToValueMap[chars[i]] = i;
        }
    }

    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        auto id = urls.size() + 1;
        urls[id] = longUrl;
        
        std::vector<char> digits;
        while(id != 0) {
            digits.emplace_back(chars[id%base]);
            id /= base;
        }
        return baseUrl + string(digits.rbegin(), digits.rend());
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        auto idString = shortUrl.substr(baseUrlSize,shortUrl.size() - baseUrlSize);
        auto id = NULL;
        auto idStringSize = idString.size();
        for(string::size_type i = 0; i < idStringSize; i++) {
            id += charToValueMap[idString[i]] * static_cast<uint64_t>(std::pow(base, idStringSize - i - 1));
        }
        return urls[id];
    }
private:
    const string chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const int base = chars.size();
    const string baseUrl = "http://tinyurl.com/";
    const int baseUrlSize= baseUrl.size();
    
    std::unordered_map<char, int> charToValueMap;
    std::unordered_map<uint64_t, string> urls;
};

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

Source: https://xenstalker.tw/2017/05/07/leetcode-problems-535-encode-and-decode-tinyurl/