question:

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.

ideas :

We can use the hash algorithm to store Key and value, key: http://tinyurl.com + random number, value: the LongURL, the key point is how to produce 6 random numbers,We have two methods, the first is to generate a random number using an array containing uppercase and lowercase letters and numbers, and the second is to generate a random number with reference to the Ascll table.

one

public static String verifyCode(){
    String str = "";
    char[] ch = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K',
              'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
            'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
    Random random = new Random();
    for (int i = 0; i <6; i++){
        char num = ch[random.nextInt(ch.length)];
        str += num;
    }
    return str;

Two

  public static String verifyCode() {
    Random random = new Random();
    String str = "";  //为了减少对象的生成，使用StringBuffer更好
   for (int i = 0; i < 6; i++){
       int key = random.nextInt(3);
       switch (key){
           case 0:
               int code1 = random.nextInt(10);
               str += code1;
               break;
           case 1:
               char code2 = (char)(random.nextInt(26)+65);
               str += code2;
               break;
           case 2:
               char code3 = (char)(random.nextInt(26)+97);
               str += code3;
               break;
       }
   }
   return str;

final：

 public class Codec {

// Encodes a URL to a shortened URL.
//储存long和shorturl
 HashMap<String,String > map =new HashMap<String ,String >();
 String  prefix ="http://tinyurl.com/" ;

public String encode(String longUrl) {
    Random random = new Random();
    String code ="";
    for (int i = 0; i < 6; i++){
        int key=random.nextInt(3);
        switch (key){
            case 0:
                int code1 = random.nextInt(10);
                code += code1;
                break;
            case 1:
                char code2 = (char)(random.nextInt(26)+65);
                code += code2;
                break;
            case 2:
                char code3 = (char)(random.nextInt(26)+97);
                code += code3;
                break;
        }
    }
    map.put(code,longUrl);
    return prefix+code;
}

// Decodes a shortened URL to its original URL.
public String decode(String shortUrl) {
    String shortCode=shortUrl.substring(shortUrl.length()-6);
 return map.get(shortCode);
}