長url與短url之間創建映射關係 Encode and Decode TinyURL

問題：

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.

解決：

① 至關於一個長字符串和短字符串 之間創建一一對應的關係，其實就至關於字符和index之間的對應關係。。

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));