dsa7 min read
Surrounded Regions — Boundary DFS Trick (LC 130)
Capture every region of Os surrounded by Xs by inverting the problem — flood from the boundary instead of the interior. The classic boundary-DFS pattern interviewers love.
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Boundary-dfs
2 articles
dsa8 min read
Pacific Atlantic Water Flow — Reverse DFS from Two Oceans (LC 417)
Find every cell from which water can flow to both oceans by reversing the flow and running DFS inward from each ocean. The reverse-flow trick that turns an O(N^4) brute force into O(N^2).
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