Medium data structure and algorithm practice puzzle.
Given a line of customers each paying 5, 10, or 20 for a 5 drink, and starting with no change, return true if you can give everyone the right change. For a 10, give back a 5. For a 20, give a 10 and a 5 if you can, and only use three 5s if you have to. Fives are the most useful, so spend them last.
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 Slot Packer, a pipeline you can run and download. See every Software Engineer project.
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