Medium data structure and algorithm practice puzzle.
Given n coins, return how many complete rows you can build when row 1 needs 1 coin, row 2 needs 2, and so on. k rows need k(k + 1) / 2 coins. Binary search for the biggest k where that's n or less.
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 Login Bisector, a pipeline you can run and download. See every Software Engineer project.
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