Hard data structure and algorithm practice puzzle.
Given an array, sort it with merge sort. Split it in half, sort each half the same way, then merge the two sorted halves back together. An array of one item is already sorted. It's O(n log n) even in the worst case, and it's STABLE, meaning equal values keep their original order.
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 Trade Leaderboard, a pipeline you can run and download. See every Software Engineer project.
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