Leetcode daily question - 535 Encryption and decryption of tinyurl

1. Title Description

535. TinyURL Encryption and decryption

TinyURL It's a kind of URL Simplify services , such as ： When you enter a URL https://leetcode.com/problems/design-tinyurl when , It will return a simplified URL http://tinyurl.com/4e9iAk . Please design a class to encrypt and decrypt TinyURL .

There is no limit to how encryption and decryption algorithms are designed and operated , You just need to guarantee one URL Can be encrypted into a TinyURL , And this TinyURL It can be restored to the original by decryption URL .

Realization Solution class ：

Solution() initialization TinyURL System object .
String encode(String longUrl) return longUrl Corresponding TinyURL .
String decode(String shortUrl) return shortUrl The original URL . The subject data guarantees a given shortUrl Is encrypted by the same system object .
Example ：

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

 explain ：
Solution obj = new Solution();
string tiny = obj.encode(url); //  Returns the result of encryption  TinyURL .
string ans = obj.decode(tiny); //  Returns the original after decryption  URL .

2. Problem solving ideas and codes

2.1 Their thinking

This problem is to generate short links and parse short links . The title does not require short links , Then we can map by using a hash table . First, when coding, start from a-z、A-Z、0-9 The short link code is generated by randomly selecting six bits from , Then judge whether the code has been generated , If it already exists, regenerate , If it does not exist, the code and the original long link will be placed in map in . The decoding part takes out the encoding according to the short link , And according to the code from map Take out the long link .

2.2 Code

public class Codec {
    
    private Map<String, String> map = new HashMap<>();
    private final static String TINY_URL = "http://tinyurl.com/";
    private final static String RANDOM_POOL = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";

    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
    
        String encode = null;
        do {
    
            encode = generate();
        } while (map.containsKey(encode));
        map.put(encode,longUrl);
        return TINY_URL+encode;
    }

    private String generate() {
    
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < 6; i++) {
    
            builder.append(RANDOM_POOL.charAt((int) (Math.random() * RANDOM_POOL.length())));
        }
        return builder.toString();
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
    

        return map.get(shortUrl.substring(TINY_URL.length()));
    }
}

2.3 test result

Pass the test

3. summary

• Use random numbers to generate six bit short link codes
• Use hash table to cache encoding and mapping of long links