Easy data structure and algorithm practice puzzle.
Given an array of digits that spells out a number, add one and return the new digits. Start at the last digit. If it's below 9, add one and you're done. If it's 9, it becomes 0 and the carry moves left. If the carry runs off the front, as with 999, put a 1 at the start. Big number libraries store numbers as digit arrays for exactly this reason.
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 Reading Range Service, a pipeline you can run and download. See every Software Engineer project.
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