leetcode535. Encode and Decode TinyURL

题目要求

Note: This is a companion problem to the System Design problem:Design TinyURL-System/).

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.

要求将长URL转化为短URL，即通过长URL可以生成短URL，短URL也可以找到长URL。

思路和代码

通过Map结构就可以实现，只需要将长URL和短URL之间的映射分别进行存储即可。

private Map<String, String> longToShortUrl = new HashMap<>();  
private Map<String, String> shortToLongUrl = new HashMap<>();  
  
private static final String SHORT\_URL\_PREFIX \= "http://tinyurl.com/";  
  
private static final String AVAILABLE_CHARACTERS = "1234567890qwertyuiopasdfghjklzxcvbnm";  
  
private Random random = new Random();  
// Encodes a URL to a shortened URL.  
public String encode(String longUrl) {  
    if (longToShortUrl.containsKey(longUrl)) {  
        return SHORT_URL_PREFIX + longToShortUrl.get(longUrl);  
    }  
    String result;  
    do{  
        result = getRandomShortUrl(6);  
    } while (shortToLongUrl.containsKey(result));  
    longToShortUrl.put(longUrl, result);  
    shortToLongUrl.put(result, longUrl);  
    return SHORT_URL_PREFIX + result;  
}  
  
// Decodes a shortened URL to its original URL.  
public String decode(String shortUrl) {  
    String shortRandomCharacters = shortUrl.replace(SHORT_URL_PREFIX, "");  
    return shortToLongUrl.get(shortRandomCharacters);  
}  
  
private String getRandomShortUrl(int length) {  
    StringBuilder sb = new StringBuilder();  
    while (length-- > 0) {  
        int randomIndex = (int)(Math.random() * AVAILABLE_CHARACTERS.length());  
        sb.append(AVAILABLE_CHARACTERS.charAt(randomIndex));  
    }  
    return sb.toString();  
}