Hard data structure and algorithm practice puzzle.
Given readings and a window size k, return the average of every run of k readings in a row, rounded down. Re-adding each window from scratch is O(n x k). Instead, keep one running sum: add the reading that enters the window and subtract the one that leaves. That's a sliding window, and it makes the whole thing O(n).
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 Event Smoothing Rig, a pipeline you can run and download. See every Software Engineer project.
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