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 a class to encode a URL and decode a tiny URL.

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.

Implement the Solution class:

Solution() Initializes the object of the system.
String encode(String longUrl) Returns a tiny URL for the given longUrl.
String decode(String shortUrl) Returns the original long URL for the given shortUrl.
It is guaranteed that the given shortUrl was encoded by the same object.

Example 1:

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

Explanation:
Solution obj = new Solution();
string tiny = obj.encode(url); // returns the encoded tiny url.
string ans = obj.decode(tiny); // returns the original url after deconding it.

Constraints:

1 <= url.length <= 104
url is guranteed to be a valid URL.

Solution

public class Codec {
    char[] symbols = new char[256];
    int[] freq = new int[256];
    String[] huffman = new String[256];
    Node root = null;

    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        for (char c: longUrl.toCharArray()) {
            freq[c]++;
            symbols[c] = c;
        }
        
        Queue<Node> pq = new PriorityQueue<>((a,b) -> {
           return Integer.compare(a.val, b.val);
        });
        
        for (int i = 0; i < freq.length; i++) {
            if (freq[i] > 0) {
                pq.add(new Node(symbols[i], freq[i]));
            }
        }
        root = null;
        while (pq.size() > 1) {
            Node left = pq.poll();
            Node right = pq.poll();
            Node node = new Node(' ', left.val + right.val, left, right);
            root = node;
            pq.add(node);
        }
        
        createHuffman(root, "");
        
        StringBuilder sb = new StringBuilder("http://tinyurl.com/");
        for (char c: longUrl.toCharArray()) {
            sb.append(huffman[c]);
        }
        return sb.toString();
    }
    
    void createHuffman(Node node, String val) {
        if (node.left == null && node.right == null) {
            huffman[node.c] = val;
            return;
        }
        
        createHuffman(node.left, val + "0");
        createHuffman(node.right, val + "1");
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        int len = shortUrl.length();
        StringBuilder url = new StringBuilder();
        
        // {0..18} is "http://tinyurl.com/" so ignore it during decoding
        for (int i = 19; i < len;) {
            Node node = root;
            while (node.left != null && node.right != null) {
                if (shortUrl.charAt(i) == '0') {
                    node = node.left;
                } else {
                    node = node.right;
                }
                i++;
            }
            url.append(node.c);
        }
        return url.toString();
    }
    
    class Node {
        char c;
        int val;
        Node left;
        Node right;
        
        public Node(char c, int val) {
            this.c = c;
            this.val = val;
        }
        public Node(char c, int val, Node left, Node right) {
            this.c = c;
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
}

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

Solution#

Solution