LeetCode 535. Encode and Decode TinyURL

题目描述：

TinyURL is a URL shortening service where you enter a URL such as https://leetcode.com/problems/design-tinyurl and it returns a short URL such as http://tinyurl.com/4e9iAk.

Design the encode and decode methods for the TinyURL service. There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL.

Note: Do not use class member/global/static variables to store states. Your encode and decode algorithms should be stateless.

短链接的维护，但不限制内部如何生成短链接。就是Hash表的问题，我直接使用的unordered_map容器所提供的Hash函数对原链接进行处理，得到一个数值，然后将该数值转换为62进制字符串（10个数字+大小写字母各26个），该字符串作为短链接的后缀部分。

class Solution {
    string tinyUrlPrefix = "http://tinyurl.com/";
    unordered_map<string, string> urls;
public:

    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        auto hashFunc = urls.hash_function();
        size_t key = hashFunc(longUrl);
        string shortUrl = tinyUrlPrefix + convertToSixtyTwoBase(key);
        urls[shortUrl] = longUrl;
        return shortUrl;
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        return urls[shortUrl];
    }
    
    string convertToSixtyTwoBase (size_t key) {
        string str;
        while (key > 0) {
            int mod = key % 62;
            if (mod < 10) str.push_back(mod + '0');
            else if (mod < 36) str.push_back(mod - 10 + 'a');
            else str.push_back(mod - 36 + 'A');
            key /= 62;
        }
        return str;
    }
};

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