Hard data structure and algorithm practice puzzle.
Given a row of asteroids, where positive ones fly right and negative ones fly left, return what's left after every crash. When two meet, the smaller one explodes, and if they're the same size, both do. Keep a stack of survivors. Each left-flying asteroid has to fight through the right-flying ones on top of the stack until it loses, wins through, or ties.
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 Expression Evaluator, a pipeline you can run and download. See every Software Engineer project.
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