How do you solve LeetCode 130 Surrounded Regions?

Use DFS from borders to mark connected 'O's as 'E', then flip remaining 'O's to 'X' and 'E's back to 'O'.

What is the time complexity for LeetCode 130?

O(m*n) — visit each cell at most once.

Why use DFS instead of BFS for Surrounded Regions?

Either works; DFS simpler recursively, same complexity. BFS if stack depth worries you.

LeetCode 130 Unlocked: The DFS Flood Fill That Saves Border O's First

Imagine acing that FAANG interview because you visualized LeetCode 130's genius twist. This surrounded regions puzzle isn't just code—it's a flood fill revelation for tomorrow's AI pathfinders.

theAIcatchup Apr 09, 2026 4 min read

Step-by-step visual execution trace of LeetCode 130 Surrounded Regions DFS marking border-connected O's

⚡ Key Takeaways

Reverse DFS from borders marks escapes first, flipping only trapped regions. 𝕏
Visual tracers like TraceLit make grid algorithms intuitive and interview-ready. 𝕏
This flood fill pattern powers AI pathfinding and game AI territory control. 𝕏

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#DFS #DFS algorithm #algorithm visualization #coding interview #coding interview prep #dfs-flood-fill #leetcode-130 #leetcode-solutions #surrounded-regions

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Originally reported by Dev.to

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