Two Sum
Given an array of integers and a target sum Return indices of two numbers that add up to target Example: [2,7,11,15], target = 9 => [0,1]
This is often your first interview question, and it's testing something specific: can you trade space for time? The brute force is obvious, but interviewers want to see you reach for a hash map naturally. It's a great warm-up that builds the instinct you'll use in dozens of harder problems.
Meet Coco
Coco is an AI study partner, not a person. She listens while you explain your code, asks what a line does and never hands you the fix. Typing with Coco is free when you sign in, up to a daily limit. Talking with her out loud comes with the $79 pack. Anyone can open the editor and run their code.
Each locked part opens in the editor once the part before it passes.