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));