Medium data structure and algorithm practice puzzle.
Given daily temperatures, return for each day how many days until a warmer one, or 0 if none comes. Use a stack of days still waiting. When a warmer day shows up, pop every waiting day that's colder and record the gap in days. Then push today. Storing indices instead of temperatures is what lets you measure the gap.
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 Login Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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