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. 根據系統設計的Estimation of the amount of data we need to store for the next couple of years, 咱們應須要6位Base62的char來encode

2. assume 避免地址爆炸，相同的longUrl獲得相同的shortUrl, 這須要一個額外的hashMap longToShort

3. 這裏由於咱們想保證都是6位的shortURL，因此採用random generate的方法；其餘方法還能夠是編號等等

 1 public class Codec {
 2     HashMap<String, String> hashToUrl = new HashMap<String, String>();
 3     HashMap<String, String> urlToHash = new HashMap<String, String>();
 4     String tinyUrlBase = "http://tinyurl.com/";
 5     String characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
 6     Random random = new Random();
 7 
 8     // Encodes a URL to a shortened URL.
 9     public String encode(String longUrl) {
10         if (urlToHash.containsKey(longUrl))
11             return tinyUrlBase + urlToHash.get(longUrl);
12             
13         StringBuilder hash = new StringBuilder();
14         do {
15             for (int i=0; i<6; i++) {
16                 hash.append(characters.charAt(random.nextInt(characters.length())));
17             }
18         } while (hashToUrl.containsKey(hash.toString()));
19         
20         hashToUrl.put(hash.toString(), longUrl);
21         urlToHash.put(longUrl, hash.toString());
22         return tinyUrlBase + hash.toString();
23     }
24 
25     // Decodes a shortened URL to its original URL.
26     public String decode(String shortUrl) {
27         return hashToUrl.get(shortUrl.substring(tinyUrlBase.length()));
28     }
29 }
30 
31 // Your Codec object will be instantiated and called as such:
32 // Codec codec = new Codec();
33 // codec.decode(codec.encode(url));