[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 = 0;
     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 > 0)
         {
             res = dict[count % 62] + res;
             count /= 62;
         }
         while (res.size() < 6) res = "0" + res;
         m[longUrl] = res;
         idm[id] = longUrl;
         return res;
     }
     // Decodes a shortened URL to its original URL.
     string decode(string shortUrl)
     {
         int id = 0;
         for (int i = 0; i < shortUrl.size();i++)
         {
             id = 62 * 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