Medium data structure and algorithm practice puzzle.
Given n, return an n by n grid where row i has a 1 in every column up to and including i, and 0 after. That's a lower triangle of 1s. It stops each token from looking at tokens that come after it, which keeps a model from peeking at the answer during training.
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 Request Tile Exporter, a pipeline you can run and download. See every Software Engineer project.
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