Hard data structure and algorithm practice puzzle.
Given a list where every value appears twice except two, return those two, smaller first. XOR everything, and you get the two loners XORed together. Pick any 1 bit of that result, since the two must differ there. Split the list by whether each number has that bit, and XOR each group. Each group gives you one of the answers.
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 Document Presence Filter, a pipeline you can run and download. See every Software Engineer project.
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