Contains Duplicate | AlgoChronicle

Step 1

Know these before you start

Tick what you already know

Concept video

Step 2

Try it on paper

5–10 minutes, no code yet

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

Problem Statement

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

Re-solved 2026-10-09 as Day 1 of the foundations rebuild.

Input / Output

Input:  nums = [1,2,3,1]
Output: true

Input:  nums = [1,2,3,4]
Output: false

Understanding the Problem

Same "have I seen this before?" instinct as Two Sum — but here we only need membership, not indices, so a set is enough
set() (not {} — that's a dict) gives O(1) average membership checks
Early return on the first repeat; False only after the full scan

Edge Cases

Empty or single-element array → False, loop simply never triggers
Duplicate at the very end → still caught, just later

Step 6

Get it accepted

All test cases pass

Step 7

Review it so it sticks

Re-solve from memory after

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 · optimal solution

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