535. Encode and Decode TinyURL - Explanation

Prerequisites

Before attempting this problem, you should be comfortable with:

Hash Maps - Used to store bidirectional mappings between long and short URLs for O(1) lookup
String Manipulation - Required for parsing URLs, extracting indices, and constructing short URL codes
System Design Basics - Understanding trade-offs between different encoding strategies (sequential IDs vs random codes)

1. List

Intuition

The simplest way to create a URL shortener is to assign each long URL a unique integer ID based on its position in a list. The short URL is just the base URL plus this index. When decoding, we extract the index from the short URL and retrieve the original URL from the list. This approach is straightforward but the short URL length grows with the number of stored URLs.

Algorithm

Maintain a list urls to store all encoded long URLs.
encode(longUrl):
- Append longUrl to the list.
- Return "http://tinyurl.com/" followed by the index (list length - 1).
decode(shortUrl):
- Extract the index from the end of the short URL.
- Return urls[index].

class Codec:

    def __init__(self):
        self.urls = []

    def encode(self, longUrl: str) -> str:
        self.urls.append(longUrl)
        return 'http://tinyurl.com/' + str(len(self.urls) - 1)

    def decode(self, shortUrl: str) -> str:
        return self.urls[int(shortUrl.split('/')[-1])]

public class Codec {
    private List<String> urls;

    public Codec() {
        urls = new ArrayList<>();
    }

    public String encode(String longUrl) {
        urls.add(longUrl);
        return "http://tinyurl.com/" + (urls.size() - 1);
    }

    public String decode(String shortUrl) {
        int index = Integer.parseInt(shortUrl.substring(shortUrl.lastIndexOf('/') + 1));
        return urls.get(index);
    }
}

class Solution {
public:
    vector<string> urls;

    string encode(string longUrl) {
        urls.push_back(longUrl);
        return "http://tinyurl.com/" + to_string(urls.size() - 1);
    }

    string decode(string shortUrl) {
        int index = stoi(shortUrl.substr(shortUrl.find_last_of('/') + 1));
        return urls[index];
    }
};

class Codec {
    constructor() {
        this.urls = [];
    }

    /**
     * Encodes a URL to a shortened URL.
     *
     * @param {string} longUrl
     * @return {string}
     */
    encode(longUrl) {
        this.urls.push(longUrl);
        return 'http://tinyurl.com/' + (this.urls.length - 1);
    }

    /**
     * Decodes a shortened URL to its original URL.
     *
     * @param {string} shortUrl
     * @return {string}
     */
    decode(shortUrl) {
        const index = parseInt(shortUrl.split('/').pop());
        return this.urls[index];
    }
}

public class Codec {
    private List<string> urls = new List<string>();

    public string encode(string longUrl) {
        urls.Add(longUrl);
        return "http://tinyurl.com/" + (urls.Count - 1);
    }

    public string decode(string shortUrl) {
        int index = int.Parse(shortUrl.Substring(shortUrl.LastIndexOf('/') + 1));
        return urls[index];
    }
}

type Codec struct {
    urls []string
}

func Constructor() Codec {
    return Codec{urls: []string{}}
}

func (this *Codec) encode(longUrl string) string {
    this.urls = append(this.urls, longUrl)
    return "http://tinyurl.com/" + strconv.Itoa(len(this.urls)-1)
}

func (this *Codec) decode(shortUrl string) string {
    parts := strings.Split(shortUrl, "/")
    index, _ := strconv.Atoi(parts[len(parts)-1])
    return this.urls[index]
}

class Codec {
    private val urls = mutableListOf<String>()

    fun encode(longUrl: String): String {
        urls.add(longUrl)
        return "http://tinyurl.com/${urls.size - 1}"
    }

    fun decode(shortUrl: String): String {
        val index = shortUrl.substringAfterLast('/').toInt()
        return urls[index]
    }
}

class Codec {
    private var urls = [String]()

    func encode(_ longUrl: String) -> String {
        urls.append(longUrl)
        return "http://tinyurl.com/\(urls.count - 1)"
    }

    func decode(_ shortUrl: String) -> String {
        let index = Int(shortUrl.split(separator: "/").last!)!
        return urls[index]
    }
}

Time & Space Complexity

Time complexity: $O(1)$ for $encode()$ and $decode()$ .
Space complexity: $O(n * m)$

Where $n$ is the number of $longUrls$ , $m$ is the average length of the URLs.

2. Hash Map - I

Intuition

Instead of using a list with implicit indices, we use a hash map with an explicit incrementing ID. This offers more flexibility since we can use the ID as a key and don't rely on list indexing. Each new URL gets assigned the current ID, which then increments. The hash map allows constant-time lookup when decoding.

Algorithm

Maintain a hash map url_map and an integer id starting at 0.
encode(longUrl):
- Store url_map[id] = longUrl.
- Generate the short URL using the current id.
- Increment id and return the short URL.
decode(shortUrl):
- Parse the ID from the short URL.
- Return url_map[id].

class Codec:

    def __init__(self):
        self.url_map = {}
        self.id = 0

    def encode(self, longUrl: str) -> str:
        self.url_map[self.id] = longUrl
        short_url = f"http://tinyurl.com/{self.id}"
        self.id += 1
        return short_url

    def decode(self, shortUrl: str) -> str:
        url_id = int(shortUrl.split('/')[-1])
        return self.url_map[url_id]

public class Codec {
    private HashMap<Integer, String> urlMap;
    private int id;

    public Codec() {
        urlMap = new HashMap<>();
        id = 0;
    }

    public String encode(String longUrl) {
        urlMap.put(id, longUrl);
        return "http://tinyurl.com/" + id++;
    }

    public String decode(String shortUrl) {
        int urlId = Integer.parseInt(shortUrl.substring(shortUrl.lastIndexOf('/') + 1));
        return urlMap.get(urlId);
    }
}

class Solution {
    unordered_map<int, string> urlMap;
    int id;

public:
    Solution() : id(0) {}

    string encode(string longUrl) {
        urlMap[id] = longUrl;
        return "http://tinyurl.com/" + to_string(id++);
    }

    string decode(string shortUrl) {
        int urlId = stoi(shortUrl.substr(shortUrl.find_last_of('/') + 1));
        return urlMap[urlId];
    }
};

class Codec {
    constructor() {
        this.urlMap = {};
        this.id = 0;
    }

    /**
     * Encodes a URL to a shortened URL.
     *
     * @param {string} longUrl
     * @return {string}
     */
    encode(longUrl) {
        this.urlMap[this.id] = longUrl;
        return `http://tinyurl.com/${this.id++}`;
    }

    /**
     * Decodes a shortened URL to its original URL.
     *
     * @param {string} shortUrl
     * @return {string}
     */
    decode(shortUrl) {
        const urlId = parseInt(shortUrl.split('/').pop(), 10);
        return this.urlMap[urlId];
    }
}

public class Codec {
    private Dictionary<int, string> urlMap = new Dictionary<int, string>();
    private int id = 0;

    public string encode(string longUrl) {
        urlMap[id] = longUrl;
        return "http://tinyurl.com/" + id++;
    }

    public string decode(string shortUrl) {
        int urlId = int.Parse(shortUrl.Substring(shortUrl.LastIndexOf('/') + 1));
        return urlMap[urlId];
    }
}

type Codec struct {
    urlMap map[int]string
    id     int
}

func Constructor() Codec {
    return Codec{urlMap: make(map[int]string), id: 0}
}

func (this *Codec) encode(longUrl string) string {
    this.urlMap[this.id] = longUrl
    this.id++
    return "http://tinyurl.com/" + strconv.Itoa(this.id-1)
}

func (this *Codec) decode(shortUrl string) string {
    parts := strings.Split(shortUrl, "/")
    urlId, _ := strconv.Atoi(parts[len(parts)-1])
    return this.urlMap[urlId]
}

class Codec {
    private val urlMap = mutableMapOf<Int, String>()
    private var id = 0

    fun encode(longUrl: String): String {
        urlMap[id] = longUrl
        return "http://tinyurl.com/${id++}"
    }

    fun decode(shortUrl: String): String {
        val urlId = shortUrl.substringAfterLast('/').toInt()
        return urlMap[urlId]!!
    }
}

class Codec {
    private var urlMap = [Int: String]()
    private var id = 0

    func encode(_ longUrl: String) -> String {
        urlMap[id] = longUrl
        let shortUrl = "http://tinyurl.com/\(id)"
        id += 1
        return shortUrl
    }

    func decode(_ shortUrl: String) -> String {
        let urlId = Int(shortUrl.split(separator: "/").last!)!
        return urlMap[urlId]!
    }
}

Time & Space Complexity

Time complexity: $O(1)$ for $encode()$ and $decode()$ .
Space complexity: $O(n * m)$

Where $n$ is the number of $longUrls$ , $m$ is the average length of the URLs.

3. Hash Map - II

Intuition

This approach uses two hash maps to create a bidirectional mapping. One maps long URLs to short URLs, and the other maps short URLs back to long URLs. This ensures that encoding the same long URL twice returns the same short URL, avoiding duplicates. The trade-off is using more memory for the extra map.

Algorithm

Maintain two hash maps: encodeMap (longUrl to shortUrl) and decodeMap (shortUrl to longUrl).
encode(longUrl):
- If longUrl is not in encodeMap, create a new short URL using the current count.
- Store the mapping in both encodeMap and decodeMap.
- Return the short URL.
decode(shortUrl):
- Look up the short URL in decodeMap and return the corresponding long URL.

class Codec:

    def __init__(self):
        self.encodeMap = {}
        self.decodeMap = {}
        self.base = "http://tinyurl.com/"

    def encode(self, longUrl: str) -> str:
        if longUrl not in self.encodeMap:
            shortUrl = self.base + str(len(self.encodeMap) + 1)
            self.encodeMap[longUrl] = shortUrl
            self.decodeMap[shortUrl] = longUrl
        return self.encodeMap[longUrl]

    def decode(self, shortUrl: str) -> str:
        return self.decodeMap[shortUrl]

public class Codec {
    private HashMap<String, String> encodeMap = new HashMap<>();
    private HashMap<String, String> decodeMap = new HashMap<>();
    private String base = "http://tinyurl.com/";

    public String encode(String longUrl) {
        if (!encodeMap.containsKey(longUrl)) {
            String shortUrl = base + (encodeMap.size() + 1);
            encodeMap.put(longUrl, shortUrl);
            decodeMap.put(shortUrl, longUrl);
        }
        return encodeMap.get(longUrl);
    }

    public String decode(String shortUrl) {
        return decodeMap.get(shortUrl);
    }
}

class Solution {
private:
    unordered_map<string, string> encodeMap;
    unordered_map<string, string> decodeMap;
    string base = "http://tinyurl.com/";

public:
    string encode(string longUrl) {
        if (encodeMap.find(longUrl) == encodeMap.end()) {
            string shortUrl = base + to_string(encodeMap.size() + 1);
            encodeMap[longUrl] = shortUrl;
            decodeMap[shortUrl] = longUrl;
        }
        return encodeMap[longUrl];
    }

    string decode(string shortUrl) {
        return decodeMap[shortUrl];
    }
};

class Codec {
    constructor() {
        this.encodeMap = new Map();
        this.decodeMap = new Map();
        this.base = 'http://tinyurl.com/';
    }

    /**
     * Encodes a URL to a shortened URL.
     *
     * @param {string} longUrl
     * @return {string}
     */
    encode(longUrl) {
        if (!this.encodeMap.has(longUrl)) {
            const shortUrl = this.base + (this.encodeMap.size + 1);
            this.encodeMap.set(longUrl, shortUrl);
            this.decodeMap.set(shortUrl, longUrl);
        }
        return this.encodeMap.get(longUrl);
    }

    /**
     * Decodes a shortened URL to its original URL.
     *
     * @param {string} shortUrl
     * @return {string}
     */
    decode(shortUrl) {
        return this.decodeMap.get(shortUrl);
    }
}

public class Codec {
    private Dictionary<string, string> encodeMap = new Dictionary<string, string>();
    private Dictionary<string, string> decodeMap = new Dictionary<string, string>();
    private string baseUrl = "http://tinyurl.com/";

    public string encode(string longUrl) {
        if (!encodeMap.ContainsKey(longUrl)) {
            string shortUrl = baseUrl + (encodeMap.Count + 1);
            encodeMap[longUrl] = shortUrl;
            decodeMap[shortUrl] = longUrl;
        }
        return encodeMap[longUrl];
    }

    public string decode(string shortUrl) {
        return decodeMap[shortUrl];
    }
}

type Codec struct {
    encodeMap map[string]string
    decodeMap map[string]string
    base      string
}

func Constructor() Codec {
    return Codec{
        encodeMap: make(map[string]string),
        decodeMap: make(map[string]string),
        base:      "http://tinyurl.com/",
    }
}

func (this *Codec) encode(longUrl string) string {
    if _, exists := this.encodeMap[longUrl]; !exists {
        shortUrl := this.base + strconv.Itoa(len(this.encodeMap)+1)
        this.encodeMap[longUrl] = shortUrl
        this.decodeMap[shortUrl] = longUrl
    }
    return this.encodeMap[longUrl]
}

func (this *Codec) decode(shortUrl string) string {
    return this.decodeMap[shortUrl]
}

class Codec {
    private val encodeMap = mutableMapOf<String, String>()
    private val decodeMap = mutableMapOf<String, String>()
    private val base = "http://tinyurl.com/"

    fun encode(longUrl: String): String {
        if (longUrl !in encodeMap) {
            val shortUrl = base + (encodeMap.size + 1)
            encodeMap[longUrl] = shortUrl
            decodeMap[shortUrl] = longUrl
        }
        return encodeMap[longUrl]!!
    }

    fun decode(shortUrl: String): String {
        return decodeMap[shortUrl]!!
    }
}

class Codec {
    private var encodeMap = [String: String]()
    private var decodeMap = [String: String]()
    private let base = "http://tinyurl.com/"

    func encode(_ longUrl: String) -> String {
        if encodeMap[longUrl] == nil {
            let shortUrl = base + String(encodeMap.count + 1)
            encodeMap[longUrl] = shortUrl
            decodeMap[shortUrl] = longUrl
        }
        return encodeMap[longUrl]!
    }

    func decode(_ shortUrl: String) -> String {
        return decodeMap[shortUrl]!
    }
}

Time & Space Complexity

Time complexity: $O(1)$ for $encode()$ and $decode()$ .
Space complexity: $O(n * m)$

Where $n$ is the number of $longUrls$ , $m$ is the average length of the URLs.

Common Pitfalls

Not Handling Duplicate URLs Consistently

If the same long URL is encoded multiple times, some implementations generate different short URLs each time, wasting storage. Using a bidirectional mapping (approach 3) ensures the same long URL always returns the same short URL, but this trade-off uses more memory.

Incorrect Parsing of the Short URL

When decoding, you must correctly extract the ID from the short URL. Common mistakes include not handling the base URL prefix properly, using the wrong string split method, or forgetting to convert the extracted string to an integer before lookup.

Integer Overflow with Sequential IDs

Using a simple incrementing integer ID works for small scale, but can overflow for very large numbers of URLs. In production systems, consider using larger integer types, UUIDs, or base62 encoding to handle billions of URLs while keeping short URLs compact.