Day 29MediumPractice

Duplicate Number in Array

HashSet Tracking

Problem

Find Duplicate

Logic: Using a HashSet provides O(1) average time for lookups, making the whole process O(N).

Solve it yourself before you look

Reading the solution first feels like progress, but it makes the next similar problem — and the interview version — much harder, because you skipped the part where you figure it out. Give it an honest 20–30 minutes. Stuck? Re-read the pattern, watch the concept video, or try the brute force first.

Hidden: approach · solution code

Revising this one?

→Solve it from memory before peeking
→Say the pattern out loud, not just the code
→Explain it like you would to a friend
→Try it again in a different language