Medium data structure and algorithm practice puzzle.
Given n, return 0 XOR 1 XOR 2 XOR ... XOR n. A loop works, but the answer repeats every 4 numbers. Look at n mod 4: if it's 0, the answer is n. If it's 1, the answer is 1. If it's 2, it's n + 1. If it's 3, it's 0. Spotting a pattern in the running answer beats speeding up the loop.
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 Match Flag Store, a pipeline you can run and download. See every Software Engineer project.
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