Easy data structure and algorithm practice puzzle.
Given a number, return how many 1 bits it has in binary. So 5 is 101, which has 2. Check the last bit with n & 1, add it to your count, then shift n right by one. Repeat until n is 0. CPUs have a single instruction for this, called popcount.
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 Trade Permission Mask, a pipeline you can run and download. See every Software Engineer project.
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