Starter: remember seen values

What does adding a value to the set make possible later? Goal: Store a value so a later check can recognize it.

Starter step 16 of 20Store what you have seen

A set remembers which values appeared earlier.

Before the final value, ask whether that value is already stored.

Problem Brief

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Puzzle Hints

What does adding a value to the set make possible later?
A set remembers a value when you add it. The right choice is "A later check can recognize that value" because it preserves the goal: Store a value so a later check can recognize it.
Tie your answer back to Arrays & Hashing: what data is stored, when it updates, and what condition uses it?

Asked at 12 companies

AdobeAirbnbAmazon

Contains Duplicate — HashSet

hashmap

Input: [1,2,3,1]. Use HashSet to detect duplicates

Map & Set in JavaScriptref

JavaScript provides Map for key-value pairs and Set for unique values. Both offer O(1) average lookup, insert, and delete. Unlike plain objects, Map preserves insertion order and allows any type as keys.

-new Map() — O(1) get/set/has/delete

-new Set() — O(1) add/has/delete, auto-deduplicates

-Map.get(key) returns undefined if missing

-for...of iterates Map entries as [key, value]

const map = new Map();
map.set('a', 1);    // set key-value
map.get('a');        // 1
map.has('a');        // true

const set = new Set([1, 2, 2, 3]);
set.size;            // 3 (deduped)
set.has(2);          // true

Official docs →

Map & Set in JavaScriptref

-new Map() — O(1) get/set/has/delete

-new Set() — O(1) add/has/delete, auto-deduplicates

-Map.get(key) returns undefined if missing

-for...of iterates Map entries as [key, value]

const map = new Map();
map.set('a', 1);    // set key-value
map.get('a');        // 1
map.has('a');        // true

const set = new Set([1, 2, 2, 3]);
set.size;            // 3 (deduped)
set.has(2);          // true

Official docs →

How to think: Hash Map / Setguide

You need O(1) lookups — checking if something exists, counting frequencies, or finding pairs.

1.Ask: "Am I searching for something repeatedly?" → Hash Map

2.Ask: "Do I need to check existence?" → Set

3.Ask: "Do I need to count occurrences?" → Map with value = count

4.Ask: "Do I need to find a pair that satisfies a condition?" → Store complement in Map

5.The tradeoff: O(n) extra space buys you O(1) per lookup

vs Nested loops (O(n²)): You're comparing every element against every other — a Map does it in one pass

vs Sorting (O(n log n)): You just need existence/frequency checks, not order

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