Hard data structure and algorithm practice puzzle.
Given n, return how many prime numbers are less than n. Use the Sieve of Eratosthenes. Start by marking every number from 2 up as maybe prime. For each prime you reach, cross off all of its multiples, starting at its square. Whatever's never crossed off is prime, so count those. It's much faster than testing each number on its own.
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 Reading Forecaster, a pipeline you can run and download. See every Software Engineer project.
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