leetcode 535. Encode and Decode TinyURL（对URL编解码）

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 a class to encode a URL and decode a tiny URL.

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.

Implement the Solution class:

Solution() Initializes the object of the system.
String encode(String longUrl) Returns a tiny URL for the given longUrl.
String decode(String shortUrl) Returns the original long URL for the given shortUrl. It is guaranteed that the given shortUrl was encoded by the same object.

Example 1:

Input: url = “https://leetcode.com/problems/design-tinyurl”
Output: “https://leetcode.com/problems/design-tinyurl”

Explanation:
Solution obj = new Solution();
string tiny = obj.encode(url); // returns the encoded tiny url.
string ans = obj.decode(tiny); // returns the original url after deconding it.

就是编解码，longURL进来编成短的URL，decode就是短的进来恢复成长的。没有规定的编解码方式，可自己定义。

思路：
没有规定的编解码方式，那就定义最简单的id，每个longURL给不同的id，从0开始，依次往上加。
用一个HashMap映射即可。

public class Codec {
    HashMap<String, String> map = new HashMap<>();
    int idx = 0;
    
    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        String shortUrl = "https://tinyurl//" + idx + ".com";
        map.put(shortUrl, longUrl);
        idx++;
        return shortUrl;
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        return map.get(shortUrl);
    }
}