535. Encode and Decode 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.

Solution：Map(id -> url)

思路：
最简单的一种编码就是用个计数器，当前是第几个存入的url就编码成几，然后解码的时候也能根据数字来找到原来的url
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

public class Codec {
    List 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