Easy data structure and algorithm practice puzzle.
Given a lowercase string and a number k, shift every letter forward k places in the alphabet, wrapping from z back to a. So "xyz" shifted by 3 is "abc". Turn each letter into 0 to 25, add k, take the remainder mod 26, and turn it back into a letter. Julius Caesar used it, and it's the cleanest way to practice mod math on characters.
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 Login Syntax Gate, a pipeline you can run and download. See every Software Engineer project.
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