Leetcode: Encode and Decode TinyURL

Note: This is a companion problem to the System Design problem: Design 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.

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1. 根据系统设计的Estimation of the amount of data we need to store for the next couple of years, 我们应需要6位Base62的char来encode

2. assume 避免地址爆炸，相同的longUrl得到相同的shortUrl, 这需要一个额外的hashMap longToShort

3. 这里因为我们想保证都是6位的shortURL，所以采用random generate的方法；其他方法还可以是编号等等

 public class Codec {
     HashMap<String, String> hashToUrl = new HashMap<String, String>();
     HashMap<String, String> urlToHash = new HashMap<String, String>();
     String tinyUrlBase = "http://tinyurl.com/";
     String characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
     Random random = new Random();

     // Encodes a URL to a shortened URL.
     public String encode(String longUrl) {
         if (urlToHash.containsKey(longUrl))
             return tinyUrlBase + urlToHash.get(longUrl);

         StringBuilder hash = new StringBuilder();
         do {
             for (int i=0; i<6; i++) {
                 hash.append(characters.charAt(random.nextInt(characters.length())));
             }
         } while (hashToUrl.containsKey(hash.toString()));

         hashToUrl.put(hash.toString(), longUrl);
         urlToHash.put(longUrl, hash.toString());
         return tinyUrlBase + hash.toString();
     }

     // Decodes a shortened URL to its original URL.
     public String decode(String shortUrl) {
         return hashToUrl.get(shortUrl.substring(tinyUrlBase.length()));
     }
 }

 // Your Codec object will be instantiated and called as such:
 // Codec codec = new Codec();
 // codec.decode(codec.encode(url));