Easy data structure and algorithm practice puzzle.
Given two numbers, swap them without a temporary variable, using XOR three times: x = x ^ y, then y = x ^ y, then x = x ^ y. Return [x, y]. It's a famous trick, but don't ship it. A temp variable is just as fast, and this trick wipes the value to 0 if both names point to the same variable.
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 Integrity Check, a pipeline you can run and download. See every Software Engineer project.
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