Hard data structure and algorithm practice puzzle.
Given labeled points, a query point, and k, return the label the k closest points vote for. Measure the distance from the query to every point, sort by distance, and take the k closest. Count their labels and return the most common one. k-nearest neighbors has no training step. It just keeps all the data, so every prediction scans the whole set.
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 Sample Ranking Service, a pipeline you can run and download. See every Software Engineer project.
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