leetcode 535. Encode and Decode TinyURL（长短网址互译）

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.

说白了就是要你把长网址变成短网址，再将短网址转化成对应的长网址，而且方法不限，所以可以随意发挥，容易AC

这里长网址变成短网址采用的方法是，定义一个常量字符串0-9a-zA-Z，每次将长网址作为key存入Map时，在将对应的短网址（在那个字符串常量中随机挑去6个作为短网址）作为对应的value存入map中，在将两者对换，存入另一个map中，用于将短网址变成长网址。存入过程中会判断长网址是否出现过，以及随机生成的短网址是否出现过，在做相应处理。

代码如下

public class Codec {    Map<String,String> longUrlToShortUrl = new HashMap<>();Map<String,String> shortUrlToLongUrl = new HashMap<>();private static final String code = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";// Encodes a URL to a shortened URL.    public String encode(String longUrl) {        String encode=null;    if(longUrlToShortUrl.get(longUrl)!=null)    {    return longUrlToShortUrl.get(longUrl);    }    else    {    encode = Codec.encodeUrl();    while(shortUrlToLongUrl.get(encode)!=null)    {    encode = Codec.encodeUrl();    }    longUrlToShortUrl.put(longUrl,encode);    shortUrlToLongUrl.put(encode, longUrl);    return encode;    }    }    public static String encodeUrl(){    String encode="";for(int i=0; i<6; i++){String str = String.valueOf(code.charAt((int)(Math.random()*61)));encode = encode + str;}return encode;    }        // Decodes a shortened URL to its original URL.    public String decode(String shortUrl) {    String decode = shortUrlToLongUrl.get(shortUrl);    return decode == null ? "" : decode;    }}// Your Codec object will be instantiated and called as such:// Codec codec = new Codec();// codec.decode(codec.encode(url));