Medium data structure and algorithm practice puzzle.
Given a list of 16-bit numbers from a packet header, return its Internet checksum. Add them up. Whenever the total goes over 65535, wrap it: keep the low 16 bits and add 1. At the end, flip every bit and keep the low 16 bits. So an empty header gives 65535.
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.
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