Hard data structure and algorithm practice puzzle.
Given campaign costs and a budget, return the most campaigns you can afford. Sort the costs from cheapest to most expensive, then keep buying until the next one doesn't fit. This greedy choice is guaranteed to give the most campaigns. If you wanted the most value instead, it fails, and that's the much harder knapsack problem.
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 Calendar Merger, a pipeline you can run and download. See every Software Engineer project.
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