Easy data structure and algorithm practice puzzle.
Given an array, move every 0 to the end and keep the other values in their original order. Pull out all the nonzero values in order, then add zeros to the end until it's back to the original length. A compacting garbage collector does the same thing, sliding live data together and leaving the free space at the end.
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 Sample Reconciler, a pipeline you can run and download. See every Software Engineer project.
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