dsa2 min read
Island Perimeter — Grid Edge Counting
Calculate the perimeter of an island in a binary grid. Each land cell contributes 4 edges minus 2 for each land neighbour.
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Max Area of Island — DFS with Area Count
Find the maximum area of any island in a binary grid. DFS returns the area of each island, track the maximum.
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Surrounded Regions — Boundary DFS
Capture all 'O' regions not connected to the border. Classic boundary DFS: mark safe cells from edges, then flip remaining O to X.
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Rotting Oranges — Multi-Source BFS
Find minimum minutes until all oranges rot. Multi-source BFS: push all initially-rotten oranges into queue simultaneously, BFS layer by layer.
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01 Matrix — Distance to Nearest 0 (Multi-Source BFS)
For each cell find distance to nearest 0. Multi-source BFS from all 0s simultaneously gives optimal O(m*n) solution.
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