Medium data structure and algorithm practice puzzle.
Given a list of stack commands, where a number is a push and "pop" removes the top, return the smallest value left on the stack at the end, or null if it's empty. Keep a second stack of minimums. On each push, push the smaller of the new value and the current min. On each pop, pop both. The current min is always on top, no scanning needed.
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 Document Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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