Dictionary

Key-Value storage — Like a real dictionary: look up a word (key), get its definition (value)

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Concept

Real-Life Analogy

** Phone book**: You look up a person's name (key) and find their phone number (value). You don't need to remember which page they're on — just search by name.

More examples:

• Configuration: Setting name (key) → setting value (value)
• ID card: ID number (key) → personal info (value)
• Cache: URL (key) → cached response (value)

Array vs Dictionary

Property	Array	Dictionary
Index	Numeric (0, 1, 2...)	Any type (string, number, object)
Lookup	By index or loop	Directly by key
Use case	Ordered lists	Key-value mapping

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Code

class Dictionary {
  constructor() {
    this.items = {}
  }

  // Set a key-value pair
  set(key, value) {
    this.items[key] = value
  }

  // Check if a key exists
  has(key) {
    return key in this.items
  }

  // Remove a key-value pair
  remove(key) {
    if (!this.has(key)) return false
    delete this.items[key]
    return true
  }

  // Get a value by key
  get(key) {
    return this.has(key) ? this.items[key] : undefined
  }

  // Get all keys
  keys() {
    return Object.keys(this.items)
  }

  // Get all values
  values() {
    return Object.values(this.items)
  }

  // Get the number of items
  size() {
    return Object.keys(this.items).length
  }

  // Clear everything
  clear() {
    this.items = {}
  }
}

Key Points

1. Wraps a JS object — this.items = {} uses JavaScript's built-in key-value feature
2. key in object checks existence — more reliable than obj[key] !== undefined
3. ** Gotcha**: Object keys are always strings. set(1, 'a') and set('1', 'a') will overwrite each other!

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Complexity

Operation	Time	Notes
set()	O(1)	Property assignment
get()	O(1)	Property access
has()	O(1)	in operator
remove()	O(1)	delete operator

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Interview Questions

1. Two Sum

Given an array of numbers and a target, find two numbers that add up to the target.

function twoSum(nums, target) {
  const dict = new Dictionary()

  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i]

    if (dict.has(complement)) {
      return [dict.get(complement), i]
    }
    dict.set(nums[i], i)
  }
  return []
}

console.log(twoSum([2, 7, 11, 15], 9)) // [0, 1]

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Summary

Property	Description
Core idea	Key-value mapping
Key ops	set (store), get (retrieve), has (check)
Time	All core ops O(1)
Limitation	Keys become strings automatically
Next	Hash Table — A faster, hand-crafted Dictionary

LeetCode Practice

#	Problem	Difficulty
1	Two Sum	Easy
242	Valid Anagram	Easy
49	Group Anagrams	Medium

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