Medium data structure and algorithm practice puzzle.
Every scan at the register logs a product code. Given the codes and k, return the k codes scanned most often, most frequent first. If two codes tie, put the smaller code first, so the report comes out the same every night.
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 Trip LRU Service, a pipeline you can run and download. See every Software Engineer project.
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