Hard data structure and algorithm practice puzzle.
Given a stack, sort it using only one extra stack, and return it from smallest at the bottom to largest on top. Pop a value from the input. While the helper's top is bigger than it, move those back to the input. Then push the value onto the helper. It's O(n squared), but the point is solving it with only stack moves.
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 Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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