Easy data structure and algorithm practice puzzle.
Given a list of numbers, apply ReLU to each one and return the results. ReLU keeps positive numbers and turns everything else into 0. It's just max(0, x). It's cheap, and it doesn't squash the signal the way sigmoid does, which is a big part of why deep networks became trainable.
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 Retention Curve, a pipeline you can run and download. See every Software Engineer project.
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