Easy data structure and algorithm practice puzzle.
Given two numbers a and b, return their greatest common divisor, the biggest number that divides both. Use Euclid's algorithm: replace the pair with the smaller number and the remainder of dividing the bigger one by it. When the remainder hits 0, the other number is your answer. It's about 2,300 years old and still the one everybody uses.
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 Calibrator, a pipeline you can run and download. See every Software Engineer project.
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