Hard data structure and algorithm practice puzzle.
Given n, return the n-bit Gray code sequence, where each number differs from the one before by exactly one bit. The ith value is i XOR (i shifted right by 1). So for n = 2, you get [0, 1, 3, 2]. Rotary knobs use this so a reading taken mid-click is never wildly wrong.
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 Integrity Check, a pipeline you can run and download. See every Software Engineer project.
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