Medium data structure and algorithm practice puzzle.
Given coin values from biggest to smallest and an amount, return how many coins the greedy approach uses, or -1 if it can't make the amount exactly. Use as many of the biggest coin as fit, then move to the next. That works for real money, but not always: with coins 4, 3, and 1, greedy makes 6 as 4 + 1 + 1 when 3 + 3 is better.
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 Matcher, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles