Hard data structure and algorithm practice puzzle.
Given a tree where every parent should equal the sum of its two children, return how many parents don't. Only nodes with both children get checked. Recurse into both sides, then check the node itself. Real Merkle trees use a hash instead of a sum, but it works the same way, and it's how git and ZFS spot corrupted data without rereading everything.
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.
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