Easy data structure and algorithm practice puzzle.
Given a number, return it with its lowest 1 bit turned off and everything else the same. So 12 (1100) becomes 8 (1000). Use n & (n - 1). Subtracting 1 flips the lowest 1 to a 0 and every 0 below it to a 1, so the AND clears just that bit.
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 Match Integrity Check, a pipeline you can run and download. See every Software Engineer project.
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