Medium data structure and algorithm practice puzzle.
Given a list of MIDI notes, return how many land on black piano keys. Take each note mod 12 to get its pitch class. The black keys are pitch classes 1, 3, 6, 8, and 10, so count the notes that land on one of those. Mod 12 works for every octave at once.
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 Hot Track Monitor, a pipeline you can run and download. See every Software Engineer project.
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