Hard data structure and algorithm practice puzzle.
Given a list of numbers and a target, return every combination that adds up to the target, where each number can be used as many times as you want. Recurse with the amount left to reach. When you pick a number, you're allowed to pick the SAME one again, but never one before it in the list. That's what stops [2, 3] and [3, 2] from both showing up.
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 Sample Audit Trail, a pipeline you can run and download. See every Software Engineer project.
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