Medium data structure and algorithm practice puzzle.
Given a grid of land ("1") and water ("0"), return the size of the biggest island, counted in cells. Land connects up, down, left, and right. Scan the grid, and every time you hit new land, flood fill the whole island while counting its cells. Keep the largest count. If there's no land, return 0.
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 Reachability Map, a pipeline you can run and download. See every Software Engineer project.
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