Coding

Hash maps

Trade space for O(1) lookups; complement / seen-set tricks.

Coding pattern

Overview

Hash maps trade memory for speed, turning repeated lookups from O(n) scans into O(1) queries. The skill is spotting when a problem is secretly asking 'have I seen this before?' and modeling it with the right key so a second nested loop disappears.

How it works

Coding pattern

ClientDataServiceEdge

Step by step, with examples

1
Keys/values
- Identify what you need to look up in O(1).
2
Hash table
- Insert entries as you scan; test membership cheaply.
3
Complement
- Query the map instead of nesting a second loop.
4
Answer
- Emit a pair, count, or grouping.
- Example: Two Sum, group anagrams

When to reach for it

'Have I seen this?' questions
Counting
Grouping by key

Example problem

Two Sum — find indices that add to a target.

Approach

Store value → index as you scan
Look up target − num before inserting

Solution

function twoSum(nums, target) {
  const seen = new Map();
  for (let i = 0; i < nums.length; i++) {
    if (seen.has(target - nums[i])) return [seen.get(target - nums[i]), i];
    seen.set(nums[i], i);
  }
}

Complexity

Time O(n), Space O(n).

Common pitfalls

Using the same element twice
Hash collisions in worst case

Where this content comes from

For full transparency, this content is curated and verified from these sources:

Curated company-tagged problem banksRecurring interview pattern librariesOppZen-authored drills & solutions