Depth-First Search/Surrounded RegionsMedium

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Surrounded Regions

Depth-First Search·Medium

Given an m x n matrix board containing 'X' and 'O', capture all regions that are 4-directionally surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region. A region is surrounded iff none of its 'O's touch the border. Equivalently: an 'O' survives iff it is connected to a border-'O'.

Mutate board in place and return it so the test runner can compare via deep equality.

Examples

Input: board = [["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]]

Output: [["X","X","X","X"],["X","X","X","X"],["X","X","X","X"],["X","O","X","X"]]

Input: board = [["X"]]

Output: [["X"]]

Constraints

m == board.length
n == board[i].length
1 <= m, n <= 200
board[i][j] is 'X' or 'O'.

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Tests

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Passed32 ms

First attempt — retry to track metric movement here.

Your approach

Pattern:Depth-First Search
Time:O(N)
Space:O(1)

Complete approach — pattern and complexity both named.

Coach

What you got right

You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.

Where it diverged

No divergence detected. The code correctly aligns with the stated approach.

Next attempt: focus on

Try solving related sliding window problems to build familiarity with two-pointer variants.

Signals

No signals fired — clean run, you stayed in flow.