Hard data structure and algorithm practice puzzle.
A floor is mapped as a grid of clean (0) and spill (1) cells, where spills connect up, down, left, and right. Any spill that touches the outer edge has drained away. Return how many spills are completely sealed inside the floor without touching an edge.
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 Frame Blast Radius, a pipeline you can run and download. See every Software Engineer project.
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