Medium data structure and algorithm practice puzzle.
Given n, return an array with the number of 1 bits in every number from 0 to n. Reuse earlier answers: the count for i is the count for i shifted right by one, plus i's last bit. So each answer takes one step, and the whole thing is O(n). That's dynamic programming in one line.
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 Step Presence Filter, a pipeline you can run and download. See every Software Engineer project.
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