Easy data structure and algorithm practice puzzle.
Given n different numbers from 0 to n, with exactly one missing, return the missing one. The numbers 0 to n add up to n(n + 1) / 2. Subtract every number you actually have, and what's left is the one that's missing. One pass, no extra array. Think of raffle tickets with one stub lost behind the booth.
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 Integrity Check, a pipeline you can run and download. See every Software Engineer project.
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