长url与短url之间创建映射关系 Encode and Decode TinyURL

问题：

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.

解决：

① 至关于一个长字符串和短字符串 之间创建一一对应的关系，其实就至关于字符和index之间的对应关系。。

public class Codec { //6ms
    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 index = Integer.valueOf(shortUrl);
        return index < urls.size() ? urls.get(index) : "";
    }
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.decode(codec.encode(url));