[leetcode-535-Encode and Decode TinyURL]

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.

思路：
encode : 将网址映射到一个id上,使用hash map,保存对应的id和网址。

decode：用短网址计算出id，然后返回对应的长网址。

string dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

     int id = ;

     unordered_map<string, string>m;

     unordered_map<int, string>idm;

     // Encodes a URL to a shortened URL.

     string encode(string longUrl)

     {

         if (m.find(longUrl) != m.end())return m[longUrl];

         string res = "";

         id++;

         int count = id;

         while (count > )

         {

             res = dict[count % ] + res;

             count /= ;

         }

         while (res.size() < ) res = "" + res;

         m[longUrl] = res;

         idm[id] = longUrl;

         return res;

     }

     // Decodes a shortened URL to its original URL.

     string decode(string shortUrl)

     {

         int id = ;

         for (int i = ; i < shortUrl.size();i++)

         {

             id =  * id + (int)(dict.find(shortUrl[i]));

         }

         if (idm.find(id) != idm.end())return idm[id];

         return "";

     }

参考：

https://discuss.leetcode.com/topic/81588/c-solution