Medium data structure and algorithm practice puzzle.
Given n different numbers from 0 to n, with exactly one missing, return the missing one. XOR together every index, every value, and n itself. Each number that's there shows up twice and cancels out, so only the missing one is left. One pass, no extra memory, and unlike adding, it can't overflow.
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 Frame Permission Mask, a pipeline you can run and download. See every Software Engineer project.
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