public class Codec

    {

        const string alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        Dictionary<string, string> url2code = new Dictionary<string, string>();

        Dictionary<string, string> code2url = new Dictionary<string, string>();

        const string TINYURL = "http://tinyurl.com/";

        // Encodes a URL to a shortened URL

        public string encode(string longUrl)

        {

            while (!url2code.ContainsKey(longUrl))//原来没有这个url的short版本

            {

                //需要生成一个新的

                var random = new Random(DateTime.Now.Millisecond);

                StringBuilder sb = new StringBuilder();

                while (sb.Length < )

                {

                    var index = random.Next();

                    sb.Append(alphabet[index]);

                }

                var code = TINYURL + sb.ToString();

                if (!code2url.ContainsKey(code))//新生成的这个code，之前没用过

                {

                    url2code.Add(longUrl, code);

                    code2url.Add(code, longUrl);

                }

            }

            return url2code[longUrl];

        }

        // Decodes a shortened URL to its original URL.

        public string decode(string shortUrl)

        {

            if (code2url.ContainsKey(shortUrl))

            {

                return code2url[shortUrl];

            }

            else

            {

                return shortUrl;

            }

        }

    }

// Your Codec object will be instantiated and called as such:

// Codec codec = new Codec();

// codec.decode(codec.encode(url));