Part 2 · Module 4
Hash map: count and look up
Use Map and Set to count frequencies, find duplicates, check anagrams, and solve two sum in one pass.
4 lessons · 47 min in total
When to reach for it
- Count with a Map
- When a problem asks "how many times", "most often", "appears once", or "same number of".
- A Set for "seen it before"
- When a problem mentions duplicates, unique, repeated, or "appears again".
- Count up, count down
- When two collections must hold the same items in any order. Anagrams, building from letters, or comparing inventories.
- Look up the partner in a map
- When a problem looks for two items that match (a sum, a difference, a pair) and O(n²) brute force is too slow.
Lessons
Module challenge
Coding problems that test every pattern in this module.