[LeetCode] 535. 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.

Thought process:
Use a simple counter.

Solution 1 (counter):
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public class Codec {
    private int i;
    private Map<Integer, String> map;
    
    public Codec() {
        i = 0;
        map = new HashMap<>();
    }

    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        map.put(i, longUrl);
        return "http://tinyurl.com/" + (i++);
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        return map.get(Integer.parseInt(shortUrl.replace("http://tinyurl.com/", "")));
    }
}

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

This solution has several issues. First, the number of tiny urls are limited by the range of integer. One might argue this can be improved by using long, but this is not the major issue. The second and major issue is that it's very easy to anticipate the next tiny urls after few tries.

A better solution is to use Java's built-in hash code function.

Solution 2 (hash code):

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public class Codec {
    Map<Integer, String> map = new HashMap<>();

    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        int hash = longUrl.hashCode();
        map.put(hash, longUrl);
        return "http://tinyurl.com/" + hash;
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        int hash = Integer.parseInt(shortUrl.replace("http://tinyurl.com/", ""));
        return map.get(hash);
    }
}

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

Time complexity:
Say the length of the tiny url is n. The time complexity of encode and decode are both O(n).
Location: Philadelphia, PA, USA

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