Hard data structure and algorithm practice puzzle.
Given coin values and an amount, return the fewest coins that add up to it exactly, or -1 if you can't. You can use each coin as often as you like. Solve every amount from 1 up: the best for an amount is 1 more than the best for amount minus some coin. Grabbing the biggest coin first doesn't always work. With coins 1, 3, and 4, it makes 6 as 4+1+1 instead of 3+3.
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 Budget Allocator, a pipeline you can run and download. See every Software Engineer project.
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