Medium data structure and algorithm practice puzzle.
Given two lists of names, return true if any name shows up in both. Put one list into a hash set, then walk the other and return true at the first name you find in the set. That's O(n + m) instead of comparing every pair, which is the same reason databases use hash joins.
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 Clause Cache Tier, a pipeline you can run and download. See every Software Engineer project.
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