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✏️Fill the BlankjavascriptEasyArrays & Hashing

Starter: return two positions

Use the saved index and the current index. Goal: Return the earlier saved position together with the position you are visiting now.

Starter step 9 of 20Move through arrays

Follow how a lookup turns into returned indexes.

The map stores an earlier index; the current loop index supplies the second answer.

Problem Brief

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice. You can return the answer in any order.

Puzzle Hints

Read the blank inside this exact statement: "return [savedIndex, ___];".
The second answer comes from the index you are visiting now. The correct token works because it preserves the goal: Return the earlier saved position together with the position you are visiting now.
Check the variable that is read immediately after the blank; that usually tells you what the blank must produce.

Asked at 72 companies

AdobeAetionAffirm

Two Sum — HashMap Lookup

hashmap

Input: [2,7,11,15], target=9. Use HashMap: complement → index

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

find pairtwo numbers thatfrequencycountduplicateanagramgroup by

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

find pairtwo numbers thatfrequencycountduplicateanagramgroup by

Drag tokens into the blanks

currentIndex

null

1// Goal: Return the earlier saved position together with the position you are visiting now.
2function starterExample() {
3  const savedIndex = 0;
4  const currentIndex = 2;
5  return [savedIndex, ___];
6}

Starter: return two positions

Use the saved index and the current index. Goal: Return the earlier saved position together with the position you are visiting now.

Starter step 9 of 20Move through arrays

Follow how a lookup turns into returned indexes.

The map stores an earlier index; the current loop index supplies the second answer.

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

1// Goal: Return the earlier saved position together with the position you are visiting now.

2function starterExample() {

3 const savedIndex = 0;

4 const currentIndex = 2;

5 return [savedIndex, ___];

6}