Hard data structure and algorithm practice puzzle.
Given a list, return every possible ordering of it. There are n factorial of them, so 10 items already gives 3.6 million. Build each ordering one spot at a time. For each spot, try every item you haven't used yet, mark it used, recurse, then unmark it. Reusing one list and undoing each choice is what backtracking means.
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 Order Stream Buffer, a pipeline you can run and download. See every Software Engineer project.
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