Easy data structure and algorithm practice puzzle.
Given an array, return true if any value appears more than once, or false if every value is different. Walk the array once and add each number to a hash set. The moment you see a number that's already in the set, return true. That's O(n) instead of comparing every pair. A database runs this exact check before it lets a row into a UNIQUE column.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Document Cache Tier, a pipeline you can run and download. See every Software Engineer project.
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