Hard data structure and algorithm practice puzzle.
Given a string of phone keypad digits from 2 to 9, return every letter combination they could spell. So "23" gives ad, ae, af, bd, and so on. Map each digit to its letters (2 is abc, 3 is def, and so on). Go one digit at a time and branch on each of its letters. An empty input returns [].
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 Step Expression Evaluator, a pipeline you can run and download. See every Software Engineer project.
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