Easy data structure and algorithm practice puzzle.
Given an array where every value appears twice except one, return the one that appears once. XOR all the values together. A number XORed with itself is 0, and 0 XORed with anything leaves it alone, so the pairs cancel out and only the single number is left. One pass, and no extra memory.
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 Flag Store, a pipeline you can run and download. See every Software Engineer project.
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