Medium data structure and algorithm practice puzzle.
Given a tree where a node holds 1 if it has a subscriber and 0 if it doesn't, return how many nodes are left after pruning. A node stays if it subscribes, or if anything below it does, since it has to pass the data along. Check the children first, then decide about the parent.
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 Packet Hierarchy Report, a pipeline you can run and download. See every Software Engineer project.
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