Medium data structure and algorithm practice puzzle.
A level editor gets commands: "place NAME" puts down a tile, "undo" takes back the last placement, and "redo" puts back the last thing undone. Return the tiles placed at the end, in order. Use two stacks. Undo moves the top of placed onto redo, and redo moves it back. Placing something new clears redo, because the history it belonged to is gone. Undo or redo with nothing to move does nothing.
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 Entity Expression Evaluator, a pipeline you can run and download. See every Software Engineer project.
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