Surrounded Regions - Codey

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Description Submissions Solution

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Solution

Updated: 2026-02-23

Idea

Only regions connected to the border should stay 'O'.

Approach

BFS/DFS from all border 'O' cells, mark them as temporary 'T'.
Flip remaining 'O' to 'X'.
Convert 'T' back to 'O'.

Why it works

An 'O' is surrounded iff it is not connected (4-dir) to any border 'O'.

Code

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Complexity

Time: O(mn)

Space: O(mn)

Solution

Updated: 2026-02-23

Idea

Only regions connected to the border should stay 'O'.

Approach

BFS/DFS from all border 'O' cells, mark them as temporary 'T'.
Flip remaining 'O' to 'X'.
Convert 'T' back to 'O'.

Why it works

An 'O' is surrounded iff it is not connected (4-dir) to any border 'O'.

Code

Loading...

Complexity

Time: O(mn)

Space: O(mn)