Medium data structure and algorithm practice puzzle.
Given a string, return every version of it with each letter in lowercase or uppercase. Digits stay the same. Go one character at a time. A digit has one option, so just add it. A letter branches two ways: lowercase first, then uppercase. Each letter doubles the number of results. Digits don't.
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 Syntax Gate, a pipeline you can run and download. See every Software Engineer project.
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