Easy data structure and algorithm practice puzzle.
Given an array, rearrange it so every even number comes before every odd one. Put one pointer at each end. Move the left one past evens and the right one past odds. When the left is on an odd and the right is on an even, swap them. It's the same move quicksort uses to split around a pivot, and it doesn't keep the original order.
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 Track Throttle, a pipeline you can run and download. See every Software Engineer project.
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