Medium data structure and algorithm practice puzzle.
Given a string, keep removing any two identical letters that sit side by side until there are none left, and return what's left. Removing a pair can make a new pair, so "abbaca" becomes "ca". Use a stack. If the new letter matches the top, pop it. Otherwise, push it.
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 Expression Evaluator, a pipeline you can run and download. See every Software Engineer project.
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