Hard data structure and algorithm practice puzzle.
Given an array, return the length of the longest strictly increasing subsequence. The numbers don't need to be next to each other. For each position, work out the longest increase that ends there: look at every earlier, smaller number and extend its best by 1. The biggest of those is the answer. That's O(n squared). There's an O(n log n) version as a follow-up.
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 Config Explorer, a pipeline you can run and download. See every Software Engineer project.
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