LeetCode - Encode and Decode TinyURL

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.

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。

public class Codec {
    private static final String dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIKKLMNOPQRSTUVWXYZ";
    Map<String,String> longToShort = new HashMap<>();
    Map<String,String> shortToLong = new HashMap<>();
    
    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        if(longToShort.containsKey(longUrl)){
            return longToShort.get(longUrl);
        }
        String encoded =  encodeString();
        while(shortToLong.containsKey(encoded)){
            encoded = encodeString();
        }
        longToShort.put(longUrl, encoded);
        shortToLong.put(encoded, longUrl);
        return encoded;
    }
    
    public String encodeString(){
        String res = "";
        for(int i = 0; i<6; i++){
            String str = String.valueOf(dict.charAt((int)(Math.random()*62)));
            res = res + str;
        }
        return res;
    }

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

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.decode(codec.encode(url));