Medium data structure and algorithm practice puzzle.
Each depot sends a list of parcel IDs already sorted from smallest to largest. Given all the lists, combine them into one sorted list. Keep duplicates, since two depots really can have the same ID. Never just glue them together and sort from scratch. Each list is already in order, so use that.
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 Event Leaderboard, a pipeline you can run and download. See every Software Engineer project.
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