Medium data structure and algorithm practice puzzle.
Given a string, compress it by writing each run of repeated letters as the letter followed by how many times it repeats. So "aaabb" becomes "a3b2" and "abc" becomes "a1b1c1". Walk the string, count while the letter stays the same, and write it out when it changes. Notice that "abc" got longer. Compression only helps when there's repetition.
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 Archive Tier, a pipeline you can run and download. See every Software Engineer project.
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