Encode and Decode TinyURL

Published at24 Jul 2026

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Design a URL shortening service like TinyURL. Implement two functions: encode(longUrl) that returns a unique short URL key, and decode(shortKey) that returns the original long URL. The short key should be a 6-character alphanumeric string. The mapping must be bijective — each long URL maps to exactly one short key, and vice versa.

A hash map is the natural choice here. We use one map to store longUrl → shortKey for encoding, and a second map to store shortKey → longUrl for decoding. When encode is called, check if the URL was already shortened. If so, return the existing key to avoid duplicates. Otherwise, generate a new 6-character key from a character set, store both mappings, and return the key.

The key generation can use a counter or random selection. Using a counter ensures uniqueness but makes keys predictable. Using random characters with a set of 62 characters (a-z, A-Z, 0-9) gives 62^6 ≈ 56 billion possibilities, making collisions extremely unlikely. For simplicity, this solution uses a counter converted to base-62 encoding.

Edge cases include decoding a key that does not exist (return an empty string), encoding the same URL twice (return the same key both times), and handling very long URLs.

Example Input & Output

Example 1

Input

"nonexistent"

Output

Explanation

Decoding an unknown key returns empty string.

Example 2

Input

"https://example.com/very/long/url/1"

Output

"a"

Explanation

Same URL returns the same key.

Example 3

Input

"https://example.com/url/3"

Output

"c"

Explanation

Third URL gets key 'c'.

Example 4

Input

"https://example.com/very/long/url/2"

Output

"b"

Explanation

Second URL gets key 'b'.

Example 5

Input

"https://example.com/very/long/url/1"

Output

"a"

Explanation

First URL gets key 'a'.

Algorithm Flow

Recommendation Algorithm Flow for Encode and Decode TinyURL

Solution Approach

Maintain two hash maps: long-to-short and short-to-long. For encode, check if already mapped. If not, generate a base-62 key using an auto-increment counter. For decode, look up the key in the short-to-long map.

function solution() {
  var map = {};
  var rev = {};
  var base = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  var id = 0;
  return {
    encode: function(url) {
      if (map[url]) return map[url];
      id++;
      var n = id;
      var key = '';
      while (n > 0) { key = base[n % 62] + key; n = Math.floor(n / 62); }
      while (key.length < 6) key = 'a' + key;
      map[url] = key;
      rev[key] = url;
      return key;
    },
    decode: function(key) {
      return rev[key] || '';
    }
  };
}

Time O(1) per operation, Space O(n).

Best Answers
These are the verified best solutions for this Challenge. You can use them as a reference to improve your solution.

java

import java.util.*;
class Solution {
    private Map<String,String> map = new HashMap<>();
    private Map<String,String> rev = new HashMap<>();
    private String base = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    private int id = 0;
    public String encode(String longUrl) {
        if (map.containsKey(longUrl)) return map.get(longUrl);
        id++;
        int n = id;
        StringBuilder key = new StringBuilder();
        while (n > 0) {
            key.insert(0, base.charAt(n % 62));
            n /= 62;
        }
        while (key.length() < 6) key.insert(0, 'a');
        String k = key.toString();
        map.put(longUrl, k);
        rev.put(k, longUrl);
        return k;
    }
    public String decode(String shortKey) {
        return rev.getOrDefault(shortKey, "");
    }
}

javascript

function solution() {
  var map = {};
  var rev = {};
  var base = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  var id = 0;
  return {
    encode: function(url) {
      if (map[url]) return map[url];
      id++;
      var n = id;
      var key = '';
      while (n > 0) { key = base[n % 62] + key; n = Math.floor(n / 62); }
      while (key.length < 6) key = 'a' + key;
      map[url] = key;
      rev[key] = url;
      return key;
    },
    decode: function(key) { return rev[key] || ''; }
  };
}

php

<?php
class Solution {
    private $map = [];
    private $rev = [];
    private $base = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    private $id = 0;
    function encode($longUrl) {
        if (isset($this->map[$longUrl])) return $this->map[$longUrl];
        $this->id++;
        $n = $this->id;
        $key = '';
        while ($n > 0) {
            $key = $this->base[$n % 62] . $key;
            $n = intdiv($n, 62);
        }
        while (strlen($key) < 6) $key = 'a' . $key;
        $this->map[$longUrl] = $key;
        $this->rev[$key] = $longUrl;
        return $key;
    }
    function decode($shortKey) {
        return isset($this->rev[$shortKey]) ? $this->rev[$shortKey] : '';
    }
}

python

class Solution:
    def __init__(self):
        self.map = {}
        self.rev = {}
        self.base = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
        self.id = 0
    def encode(self, longUrl):
        if longUrl in self.map:
            return self.map[longUrl]
        self.id += 1
        n = self.id
        key = ''
        while n > 0:
            key = self.base[n % 62] + key
            n //= 62
        while len(key) < 6:
            key = 'a' + key
        self.map[longUrl] = key
        self.rev[key] = longUrl
        return key
    def decode(self, shortKey):
        return self.rev.get(shortKey, '')

rust

use std::collections::HashMap;
struct Solution {
    map: HashMap<String, String>,
    rev: HashMap<String, String>,
    base: Vec<char>,
    id: u64,
}
impl Solution {
    fn new() -> Self {
        let base: Vec<char> = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".chars().collect();
        Solution { map: HashMap::new(), rev: HashMap::new(), base, id: 0 }
    }
    fn encode(&mut self, long_url: &str) -> String {
        let lu = long_url.to_string();
        if let Some(k) = self.map.get(&lu) { return k.clone(); }
        self.id += 1;
        let mut n = self.id;
        let mut key = String::new();
        while n > 0 {
            key.insert(0, self.base[(n % 62) as usize]);
            n /= 62;
        }
        while key.len() < 6 { key.insert(0, 'a'); }
        self.map.insert(lu.clone(), key.clone());
        self.rev.insert(key.clone(), lu);
        key
    }
    fn decode(&self, short_key: &str) -> String {
        self.rev.get(short_key).cloned().unwrap_or_default()
    }
}

typescript

function solution(): any {
    var map: any = {};
    var rev: any = {};
    var base = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    var id = 0;
    return {
        encode: function(url: string): string {
            if (map[url]) return map[url];
            id++;
            var n = id;
            var key = '';
            while (n > 0) { key = base[~~(n % 62)] + key; n = ~~(n / 62); }
            while (key.length < 6) key = 'a' + key;
            map[url] = key;
            rev[key] = url;
            return key;
        },
        decode: function(key: string): string { return rev[key] || ''; }
    };
}

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Encode and Decode TinyURL

Example Input & Output

Algorithm Flow

Solution Approach

Best AnswersThese are the verified best solutions for this Challenge. You can use them as a reference to improve your solution.

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Best Answers
These are the verified best solutions for this Challenge. You can use them as a reference to improve your solution.