CodecUrl.java

/*below is the tiny url solution in java, also this is the similar method in industry. In industry, most of shorten url service is by database, one auto increasing long number as primary key. whenever a long url need to be shorten, append to the database, and return the primary key number. (the database is very easy to distribute to multiple machine like HBase, or even you can use the raw file system to store data and improve performance by shard and replica).
Note, it's meaningless to promise the same long url to be shorten as the same short url. if you do the promise and use something like hash to check existing, the benefit is must less than the cost.
Note: if you want the shorted url contains '0-9a-zA-Z' instead of '0-9', then you need to use 62 number system, not 10 number system(decimal) to convert the primary key number. like 123->'123' in decimal, 123->'1Z' in 62 number system (or '0001Z' for align).*/

public class Codec {
    List<String> urls = new ArrayList<>();

    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        urls.add(longUrl);
        return String.valueOf(urls.size()-1);
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        int i = Integer.valueOf(shortUrl);
        return i < urls.size() ? urls.get(i) : "";
    }
}

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