Hard data structure and algorithm practice puzzle.
Given a string of digits where 1 means A and 26 means Z, return how many ways it can be decoded. At each position, you can read one digit, or two digits if they make 10 to 26. A "0" on its own can't be decoded, so a piece starting with 0 gives 0 ways. Recurse from the front and add up the ways.
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 Trip Bundle Builder, a pipeline you can run and download. See every Software Engineer project.
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