535 Encode and Decode TinyURL

Problem:

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.

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Solutions:

public class Codec {
    HashMap<String, String> longToShort = new HashMap<String, String>();
    HashMap<String, String> shortToLong = new HashMap<String, String>();
    String seed = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        if (longToShort.containsKey(longUrl)) {
            return longToShort.get(longUrl);
        }
        String res = "";
        while (true) {
            res = random();
            if (!shortToLong.containsKey(res)) {
                shortToLong.put(res, longUrl);
                longToShort.put(longUrl, res);
                break;
            }
        }
        return res;
    }
    private String random(){
        StringBuilder sb = new StringBuilder();
        Random r = new Random();
        for (int i = 0; i < 6; i ++) {
            int j = r.nextInt(62);
            sb.append(seed.charAt(j));
        }
        return sb.toString();
    }

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

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

public class Codec {
    HashMap<String, String> longToShort = new HashMap<String, String>();
    HashMap<String, String> shortToLong = new HashMap<String, String>();
    String seed = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    int nextid = 0;
    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        if (longToShort.containsKey(longUrl)) {
            return longToShort.get(longUrl);
        }
        StringBuilder sb = new StringBuilder();
        int id = nextid;
        for (int i = 0; i < 6; i ++) {
            int j = id % 62;
            id = id / 62;
            sb.insert(0, seed.charAt(j));
        }
        String res = sb.toString();
        shortToLong.put(res, longUrl);
        longToShort.put(longUrl, res);
        nextid ++;
        return res;
    }

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

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