130. Surrounded Regions - Explanation

Description

You are given a 2-D matrix board containing 'X' and 'O' characters.

If a continous, four-directionally connected group of 'O's is surrounded by 'X's, it is considered to be surrounded.

Change all surrounded regions of 'O's to 'X's and do so in-place by modifying the input board.

Example 1:

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

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

Explanation: Note that regions that are on the border are not considered surrounded regions.

Constraints:

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

Topics

Recommended Time & Space Complexity

You should aim for a solution with O(m * n) time and O(m * n) space, where m is the number of rows and n is the number of columns in the matrix.

Hint 1

We observe that we need to capture the regions that are not connected to the O's on the border of the matrix. This means there should be no path connecting the O's on the border to any O's in the region. Can you think of a way to check the region connected to these border O's?

Hint 2

We can use the Depth First Search (DFS) algorithm. Instead of checking the region connected to the border O's, we can reverse the approach and mark the regions that are reachable from the border O's. How would you implement this?

Hint 3

We run the DFS from every 'O' on the border of the matrix, visiting the neighboring cells that are equal to 'O' recursively and marking them as '#' to avoid revisiting. After completing all the DFS calls, we traverse the matrix again and capture the cells where matrix[i][j] == 'O', and unmark the cells back to 'O' where matrix[i][j] == '#'.

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Prerequisites

Before attempting this problem, you should be comfortable with:

Graph Traversal (DFS/BFS) - Exploring connected components in a 2D grid using depth-first or breadth-first search
Flood Fill Algorithm - Marking all connected cells starting from a given position
Union-Find (Disjoint Set Union) - Alternative approach using DSU to group connected components and check connectivity

1. Depth First Search

Intuition

Only the 'O' regions that touch the border can never be surrounded, because they have a path to the outside of the board.
So instead of trying to find surrounded regions directly, we do the opposite:

Mark all border-connected 'O' cells as “safe” (temporary mark 'T').
Any remaining 'O' is truly surrounded → flip it to 'X'.
Convert the temporary 'T' back to 'O'.

Algorithm

Let ROWS and COLS be board dimensions.
Define capture(r, c) (dfs):
- If out of bounds or cell is not 'O', return.
- Mark cell as 'T'.
- dfs to its 4 neighbors (up, down, left, right).
Run capture from every border cell that is 'O':
- All cells in first/last column.
- All cells in first/last row.
Scan entire board:
- If cell is 'O', it's surrounded → change to 'X'.
- If cell is 'T', it's safe → change back to 'O'.

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        ROWS, COLS = len(board), len(board[0])

        def capture(r, c):
            if (r < 0 or c < 0 or r == ROWS or
                c == COLS or board[r][c] != "O"
            ):
                return
            board[r][c] = "T"
            capture(r + 1, c)
            capture(r - 1, c)
            capture(r, c + 1)
            capture(r, c - 1)

        for r in range(ROWS):
            if board[r][0] == "O":
                capture(r, 0)
            if board[r][COLS - 1] == "O":
                capture(r, COLS - 1)

        for c in range(COLS):
            if board[0][c] == "O":
                capture(0, c)
            if board[ROWS - 1][c] == "O":
                capture(ROWS - 1, c)

        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == "O":
                    board[r][c] = "X"
                elif board[r][c] == "T":
                    board[r][c] = "O"

public class Solution {
    private int ROWS, COLS;

    public void solve(char[][] board) {
        ROWS = board.length;
        COLS = board[0].length;

        for (int r = 0; r < ROWS; r++) {
            if (board[r][0] == 'O') {
                capture(board, r, 0);
            }
            if (board[r][COLS - 1] == 'O') {
                capture(board, r, COLS - 1);
            }
        }

        for (int c = 0; c < COLS; c++) {
            if (board[0][c] == 'O') {
                capture(board, 0, c);
            }
            if (board[ROWS - 1][c] == 'O') {
                capture(board, ROWS - 1, c);
            }
        }

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X';
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O';
                }
            }
        }
    }

    private void capture(char[][] board, int r, int c) {
        if (r < 0 || c < 0 || r >= ROWS ||
            c >= COLS || board[r][c] != 'O') {
            return;
        }
        board[r][c] = 'T';
        capture(board, r + 1, c);
        capture(board, r - 1, c);
        capture(board, r, c + 1);
        capture(board, r, c - 1);
    }
}

class Solution {
    int ROWS, COLS;

public:
    void solve(vector<vector<char>>& board) {
        ROWS = board.size();
        COLS = board[0].size();

        for (int r = 0; r < ROWS; r++) {
            if (board[r][0] == 'O') {
                capture(board, r, 0);
            }
            if (board[r][COLS - 1] == 'O') {
                capture(board, r, COLS - 1);
            }
        }

        for (int c = 0; c < COLS; c++) {
            if (board[0][c] == 'O') {
                capture(board, 0, c);
            }
            if (board[ROWS - 1][c] == 'O') {
                capture(board, ROWS - 1, c);
            }
        }

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X';
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O';
                }
            }
        }
    }

private:
    void capture(vector<vector<char>>& board, int r, int c) {
        if (r < 0 || c < 0 || r >= ROWS ||
            c >= COLS || board[r][c] != 'O') {
            return;
        }
        board[r][c] = 'T';
        capture(board, r + 1, c);
        capture(board, r - 1, c);
        capture(board, r, c + 1);
        capture(board, r, c - 1);
    }
};

class Solution {
    /**
     * @param {character[][]} board
     * @return {void} Do not return anything, modify board in-place instead.
     */
    solve(board) {
        let ROWS = board.length,
            COLS = board[0].length;

        const capture = (r, c) => {
            if (
                r < 0 ||
                c < 0 ||
                r == ROWS ||
                c == COLS ||
                board[r][c] !== 'O'
            ) {
                return;
            }
            board[r][c] = 'T';
            capture(r + 1, c);
            capture(r - 1, c);
            capture(r, c + 1);
            capture(r, c - 1);
        };

        for (let r = 0; r < ROWS; r++) {
            if (board[r][0] === 'O') capture(r, 0);
            if (board[r][COLS - 1] === 'O') capture(r, COLS - 1);
        }

        for (let c = 0; c < COLS; c++) {
            if (board[0][c] === 'O') capture(0, c);
            if (board[ROWS - 1][c] === 'O') capture(ROWS - 1, c);
        }

        for (let r = 0; r < ROWS; r++) {
            for (let c = 0; c < COLS; c++) {
                if (board[r][c] === 'O') board[r][c] = 'X';
                else if (board[r][c] === 'T') board[r][c] = 'O';
            }
        }
    }
}

public class Solution {
    private int ROWS, COLS;

    public void Solve(char[][] board) {
        ROWS = board.Length;
        COLS = board[0].Length;

        for (int r = 0; r < ROWS; r++) {
            if (board[r][0] == 'O') {
                Capture(board, r, 0);
            }
            if (board[r][COLS - 1] == 'O') {
                Capture(board, r, COLS - 1);
            }
        }

        for (int c = 0; c < COLS; c++) {
            if (board[0][c] == 'O') {
                Capture(board, 0, c);
            }
            if (board[ROWS - 1][c] == 'O') {
                Capture(board, ROWS - 1, c);
            }
        }

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X';
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O';
                }
            }
        }
    }

    private void Capture(char[][] board, int r, int c) {
        if (r < 0 || c < 0 || r == ROWS ||
            c == COLS || board[r][c] != 'O') {
            return;
        }
        board[r][c] = 'T';
        Capture(board, r + 1, c);
        Capture(board, r - 1, c);
        Capture(board, r, c + 1);
        Capture(board, r, c - 1);
    }
}

func solve(board [][]byte) {
    rows, cols := len(board), len(board[0])

    var capture func(r, c int)
    capture = func(r, c int) {
        if r < 0 || c < 0 || r == rows ||
           c == cols || board[r][c] != 'O' {
            return
        }
        board[r][c] = 'T'
        capture(r+1, c)
        capture(r-1, c)
        capture(r, c+1)
        capture(r, c-1)
    }

    for r := 0; r < rows; r++ {
        if board[r][0] == 'O' {
            capture(r, 0)
        }
        if board[r][cols-1] == 'O' {
            capture(r, cols-1)
        }
    }

    for c := 0; c < cols; c++ {
        if board[0][c] == 'O' {
            capture(0, c)
        }
        if board[rows-1][c] == 'O' {
            capture(rows-1, c)
        }
    }

    for r := 0; r < rows; r++ {
        for c := 0; c < cols; c++ {
            if board[r][c] == 'O' {
                board[r][c] = 'X'
            } else if board[r][c] == 'T' {
                board[r][c] = 'O'
            }
        }
    }
}

class Solution {
    fun solve(board: Array<CharArray>) {
        val rows = board.size
        val cols = board[0].size

        fun capture(r: Int, c: Int) {
            if (r < 0 || c < 0 || r == rows ||
                c == cols || board[r][c] != 'O') {
                return
            }
            board[r][c] = 'T'
            capture(r + 1, c)
            capture(r - 1, c)
            capture(r, c + 1)
            capture(r, c - 1)
        }

        for (r in 0 until rows) {
            if (board[r][0] == 'O') {
                capture(r, 0)
            }
            if (board[r][cols - 1] == 'O') {
                capture(r, cols - 1)
            }
        }

        for (c in 0 until cols) {
            if (board[0][c] == 'O') {
                capture(0, c)
            }
            if (board[rows - 1][c] == 'O') {
                capture(rows - 1, c)
            }
        }

        for (r in 0 until rows) {
            for (c in 0 until cols) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X'
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O'
                }
            }
        }
    }
}

class Solution {
    func solve(_ board: inout [[Character]]) {
        let ROWS = board.count
        let COLS = board[0].count

        func capture(_ r: Int, _ c: Int) {
            if r < 0 || c < 0 || r == ROWS || c == COLS || board[r][c] != "O" {
                return
            }
            board[r][c] = "T"
            capture(r + 1, c)
            capture(r - 1, c)
            capture(r, c + 1)
            capture(r, c - 1)
        }

        for r in 0..<ROWS {
            if board[r][0] == "O" {
                capture(r, 0)
            }
            if board[r][COLS - 1] == "O" {
                capture(r, COLS - 1)
            }
        }

        for c in 0..<COLS {
            if board[0][c] == "O" {
                capture(0, c)
            }
            if board[ROWS - 1][c] == "O" {
                capture(ROWS - 1, c)
            }
        }

        for r in 0..<ROWS {
            for c in 0..<COLS {
                if board[r][c] == "O" {
                    board[r][c] = "X"
                } else if board[r][c] == "T" {
                    board[r][c] = "O"
                }
            }
        }
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(m * n)$

Where $m$ is the number of rows and $n$ is the number of columns of the $board$ .

2. Breadth First Search

Intuition

Same idea as DFS, but we use BFS with a queue.

Any 'O' that is connected to the border can “escape”, so it should NOT be flipped.
Start BFS from all border 'O' cells and mark every reachable 'O' as temporary 'T' (safe).
After that:
- leftover 'O' cells are fully surrounded → flip to 'X'
- 'T' cells are safe → change back to 'O'

Algorithm

Initialize a queue and push all border cells that contain 'O'.
While the queue is not empty:
- Pop a cell (r, c)
- If it is 'O', mark it as 'T'
- Push its 4 neighbors (up/down/left/right) if they are in bounds
Traverse the entire board:
- Change 'O' → 'X' (surrounded)
- Change 'T' → 'O' (safe)

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        ROWS, COLS = len(board), len(board[0])
        directions = [(1, 0), (-1, 0), (0, 1), (0, -1)]

        def capture():
            q = deque()
            for r in range(ROWS):
                for c in range(COLS):
                    if (r == 0 or r == ROWS - 1 or
                        c == 0 or c == COLS - 1 and
                        board[r][c] == "O"
                    ):
                        q.append((r, c))
            while q:
                r, c = q.popleft()
                if board[r][c] == "O":
                    board[r][c] = "T"
                    for dr, dc in directions:
                        nr, nc = r + dr, c + dc
                        if 0 <= nr < ROWS and 0 <= nc < COLS:
                            q.append((nr, nc))

        capture()
        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] == "O":
                    board[r][c] = "X"
                elif board[r][c] == "T":
                    board[r][c] = "O"

class Solution {
    int ROWS, COLS;
    vector<pair<int, int>> directions = {{1, 0}, {-1, 0},
                                         {0, 1}, {0, -1}};

public:
    void solve(vector<vector<char>>& board) {
        ROWS = board.size();
        COLS = board[0].size();

        capture(board);

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X';
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O';
                }
            }
        }
    }

private:
    void capture(vector<vector<char>>& board) {
        queue<pair<int, int>> q;
        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (r == 0 || r == ROWS - 1 ||
                    c == 0 || c == COLS - 1 &&
                    board[r][c] == 'O') {
                    q.push({r, c});
                }
            }
        }
        while (!q.empty()) {
            auto [r, c] = q.front();
            q.pop();
            if (board[r][c] == 'O') {
                board[r][c] = 'T';
                for (auto& direction : directions) {
                    int nr = r + direction.first;
                    int nc = c + direction.second;
                    if (nr >= 0 && nr < ROWS &&
                        nc >= 0 && nc < COLS) {
                        q.push({nr, nc});
                    }
                }
            }
        }
    }
};

class Solution {
    /**
     * @param {character[][]} board
     * @return {void} Do not return anything, modify board in-place instead.
     */
    solve(board) {
        let ROWS = board.length,
            COLS = board[0].length;
        let directions = [
            [1, 0],
            [-1, 0],
            [0, 1],
            [0, -1],
        ];

        const capture = () => {
            let q = new Queue();
            for (let r = 0; r < ROWS; r++) {
                for (let c = 0; c < COLS; c++) {
                    if (
                        r === 0 ||
                        r === ROWS - 1 ||
                        c === 0 ||
                        (c === COLS - 1 && board[r][c] === 'O')
                    ) {
                        q.push([r, c]);
                    }
                }
            }
            while (!q.isEmpty()) {
                let [r, c] = q.pop();
                if (board[r][c] === 'O') {
                    board[r][c] = 'T';
                    for (let [dr, dc] of directions) {
                        let nr = r + dr,
                            nc = c + dc;
                        if (nr >= 0 && nr < ROWS && nc >= 0 && nc < COLS) {
                            q.push([nr, nc]);
                        }
                    }
                }
            }
        };

        capture();
        for (let r = 0; r < ROWS; r++) {
            for (let c = 0; c < COLS; c++) {
                if (board[r][c] === 'O') board[r][c] = 'X';
                else if (board[r][c] === 'T') board[r][c] = 'O';
            }
        }
    }
}

class Solution {
    fun solve(board: Array<CharArray>) {
        val rows = board.size
        val cols = board[0].size
        val directions = arrayOf(intArrayOf(1, 0),
                                 intArrayOf(-1, 0),
                                 intArrayOf(0, 1),
                                 intArrayOf(0, -1))

        fun capture() {
            val q: LinkedList<IntArray> = LinkedList()
            for (r in 0 until rows) {
                for (c in 0 until cols) {
                    if (r == 0 || r == rows - 1 || c == 0 || c == cols - 1) {
                        if (board[r][c] == 'O') {
                            q.add(intArrayOf(r, c))
                        }
                    }
                }
            }
            while (q.isNotEmpty()) {
                val (r, c) = q.poll()
                if (board[r][c] == 'O') {
                    board[r][c] = 'T'
                    for (dir in directions) {
                        val nr = r + dir[0]
                        val nc = c + dir[1]
                        if (nr in 0 until rows && nc in 0 until cols) {
                            q.add(intArrayOf(nr, nc))
                        }
                    }
                }
            }
        }

        capture()

        for (r in 0 until rows) {
            for (c in 0 until cols) {
                if (board[r][c] == 'O') {
                    board[r][c] = 'X'
                } else if (board[r][c] == 'T') {
                    board[r][c] = 'O'
                }
            }
        }
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(m * n)$

Where $m$ is the number of rows and $n$ is the number of columns of the $board$ .

Common Pitfalls

Trying to Find Surrounded Regions Directly

The intuitive approach of finding regions completely surrounded by 'X' is error-prone. A region touching any border cell cannot be captured, and checking this condition during a flood fill is complex. The correct approach is to invert the logic: first mark all border-connected 'O' cells as safe, then flip all remaining 'O' cells. This reversal simplifies the problem significantly.

Forgetting to Check All Four Borders

When marking safe regions, you must start DFS/BFS from 'O' cells on all four borders: top row, bottom row, left column, and right column. A common mistake is only checking two opposite edges (like top and bottom) and missing cells connected through the left or right borders. Ensure your initial seeding loop covers all border cells.

Modifying Cells Without a Temporary Marker

If you flip 'O' to 'X' immediately when you find a surrounded region, you may incorrectly process cells that should remain 'O'. The standard approach uses a temporary marker (like 'T') to distinguish between safe 'O' cells and those to be processed. After marking is complete, convert 'T' back to 'O' and remaining 'O' to 'X' in a final pass.

3. Disjoint Set Union

Intuition

Treat every 'O' cell as a node in a graph. Two 'O' cells belong to the same region if they are 4-directionally connected.

The key observation:

Any region of 'O' that touches the border is safe (it cannot be surrounded).
Any region of 'O' that does not touch the border is captured → should become 'X'.

So we use DSU (Union-Find) to group connected 'O' cells, and we create one extra dummy node that represents "connected to border".

Union every border 'O' with the dummy node.
Union every 'O' with its neighboring 'O' cells.
Finally, any cell not connected to the dummy node is surrounded → flip to 'X'.

Algorithm

Create a dsu for (ROWS * COLS) cells plus 1 dummy node.
For each cell (r, c):
- If it is not 'O', skip.
- Convert (r, c) to an id: id = r * COLS + c.
- If (r, c) is on the border, union id with dummy.
- Union id with any 4-direction neighbor that is also 'O'.
Traverse the grid again:
- If a cell is 'O' but not connected to dummy, flip it to 'X'.
- Otherwise keep it as 'O'.

class DSU:
    def __init__(self, n):
        self.Parent = list(range(n + 1))
        self.Size = [1] * (n + 1)

    def find(self, node):
        if self.Parent[node] != node:
            self.Parent[node] = self.find(self.Parent[node])
        return self.Parent[node]

    def union(self, u, v):
        pu = self.find(u)
        pv = self.find(v)
        if pu == pv:
            return False
        if self.Size[pu] >= self.Size[pv]:
            self.Size[pu] += self.Size[pv]
            self.Parent[pv] = pu
        else:
            self.Size[pv] += self.Size[pu]
            self.Parent[pu] = pv
        return True

    def connected(self, u, v):
        return self.find(u) == self.find(v)

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        ROWS, COLS = len(board), len(board[0])
        directions = [(1, 0), (-1, 0), (0, 1), (0, -1)]
        dsu = DSU(ROWS * COLS + 1)

        for r in range(ROWS):
            for c in range(COLS):
                if board[r][c] != "O":
                    continue
                if (r == 0 or c == 0 or
                    r == (ROWS - 1) or c == (COLS - 1)
                ):
                    dsu.union(ROWS * COLS, r * COLS + c)
                else:
                    for dx, dy in directions:
                        nr, nc = r + dx, c + dy
                        if board[nr][nc] == "O":
                            dsu.union(r * COLS + c, nr * COLS + nc)

        for r in range(ROWS):
            for c in range(COLS):
                if not dsu.connected(ROWS * COLS, r * COLS + c):
                    board[r][c] = "X"

class DSU {
    vector<int> Parent, Size;

public:
    DSU(int n) {
        Parent.resize(n + 1);
        Size.resize(n + 1);
        for (int i = 0; i <= n; i++) {
            Parent[i] = i;
            Size[i] = 1;
        }
    }

    int find(int node) {
        if (Parent[node] != node) {
            Parent[node] = find(Parent[node]);
        }
        return Parent[node];
    }

    bool unionNodes(int u, int v) {
        int pu = find(u), pv = find(v);
        if (pu == pv) return false;
        if (Size[pu] >= Size[pv]) {
            Size[pu] += Size[pv];
            Parent[pv] = pu;
        } else {
            Size[pv] += Size[pu];
            Parent[pu] = pv;
        }
        return true;
    }

    bool connected(int u, int v) {
        return find(u) == find(v);
    }
};

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        int ROWS = board.size(), COLS = board[0].size();
        DSU dsu(ROWS * COLS + 1);
        vector<vector<int>> directions = {{1, 0}, {-1, 0},
                                          {0, 1}, {0, -1}};

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] != 'O') continue;
                if (r == 0 || c == 0 ||
                    r == ROWS - 1 || c == COLS - 1) {
                    dsu.unionNodes(ROWS * COLS, r * COLS + c);
                } else {
                    for (auto& dir : directions) {
                        int nr = r + dir[0], nc = c + dir[1];
                        if (board[nr][nc] == 'O') {
                            dsu.unionNodes(r * COLS + c, nr * COLS + nc);
                        }
                    }
                }
            }
        }

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (!dsu.connected(ROWS * COLS, r * COLS + c)) {
                    board[r][c] = 'X';
                }
            }
        }
    }
};

class DSU {
    constructor(n) {
        this.Parent = Array.from({ length: n + 1 }, (_, i) => i);
        this.Size = Array(n + 1).fill(1);
    }

    /**
     * @param {number} node
     * @return {number}
     */
    find(node) {
        if (this.Parent[node] !== node) {
            this.Parent[node] = this.find(this.Parent[node]);
        }
        return this.Parent[node];
    }

    /**
     * @param {number} u
     * @param {number} v
     * @return {boolean}
     */
    union(u, v) {
        let pu = this.find(u);
        let pv = this.find(v);
        if (pu === pv) return false;
        if (this.Size[pu] >= this.Size[pv]) {
            this.Size[pu] += this.Size[pv];
            this.Parent[pv] = pu;
        } else {
            this.Size[pv] += this.Size[pu];
            this.Parent[pu] = pv;
        }
        return true;
    }

    /**
     * @param {number} u
     * @param {number} v
     * @return {boolean}
     */
    connected(u, v) {
        return this.find(u) === this.find(v);
    }
}

class Solution {
    /**
     * @param {character[][]} board
     * @return {void} Do not return anything, modify board in-place instead.
     */
    solve(board) {
        const ROWS = board.length,
            COLS = board[0].length;
        const dsu = new DSU(ROWS * COLS + 1);
        const directions = [
            [1, 0],
            [-1, 0],
            [0, 1],
            [0, -1],
        ];

        for (let r = 0; r < ROWS; r++) {
            for (let c = 0; c < COLS; c++) {
                if (board[r][c] !== 'O') continue;
                if (r === 0 || c === 0 || r === ROWS - 1 || c === COLS - 1) {
                    dsu.union(ROWS * COLS, r * COLS + c);
                } else {
                    for (let [dx, dy] of directions) {
                        const nr = r + dx,
                            nc = c + dy;
                        if (board[nr][nc] === 'O') {
                            dsu.union(r * COLS + c, nr * COLS + nc);
                        }
                    }
                }
            }
        }

        for (let r = 0; r < ROWS; r++) {
            for (let c = 0; c < COLS; c++) {
                if (!dsu.connected(ROWS * COLS, r * COLS + c)) {
                    board[r][c] = 'X';
                }
            }
        }
    }
}

public class DSU {
    private int[] Parent, Size;

    public DSU(int n) {
        Parent = new int[n + 1];
        Size = new int[n + 1];
        for (int i = 0; i <= n; i++) {
            Parent[i] = i;
            Size[i] = 1;
        }
    }

    public int Find(int node) {
        if (Parent[node] != node) {
            Parent[node] = Find(Parent[node]);
        }
        return Parent[node];
    }

    public bool Union(int u, int v) {
        int pu = Find(u), pv = Find(v);
        if (pu == pv) return false;
        if (Size[pu] >= Size[pv]) {
            Size[pu] += Size[pv];
            Parent[pv] = pu;
        } else {
            Size[pv] += Size[pu];
            Parent[pu] = pv;
        }
        return true;
    }

    public bool Connected(int u, int v) {
        return Find(u) == Find(v);
    }
}

public class Solution {
    public void Solve(char[][] board) {
        int ROWS = board.Length, COLS = board[0].Length;
        DSU dsu = new DSU(ROWS * COLS + 1);
        int[][] directions = new int[][] {
            new int[] { 1, 0 }, new int[] { -1, 0 },
            new int[] { 0, 1 }, new int[] { 0, -1 }
        };

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (board[r][c] != 'O') continue;
                if (r == 0 || c == 0 ||
                    r == ROWS - 1 || c == COLS - 1) {
                    dsu.Union(ROWS * COLS, r * COLS + c);
                } else {
                    foreach (var dir in directions) {
                        int nr = r + dir[0], nc = c + dir[1];
                        if (board[nr][nc] == 'O') {
                            dsu.Union(r * COLS + c, nr * COLS + nc);
                        }
                    }
                }
            }
        }

        for (int r = 0; r < ROWS; r++) {
            for (int c = 0; c < COLS; c++) {
                if (!dsu.Connected(ROWS * COLS, r * COLS + c)) {
                    board[r][c] = 'X';
                }
            }
        }
    }
}

type DSU struct {
    Parent []int
    Size   []int
}

func NewDSU(n int) *DSU {
    dsu := &DSU{
        Parent: make([]int, n+1),
        Size:   make([]int, n+1),
    }
    for i := 0; i <= n; i++ {
        dsu.Parent[i] = i
        dsu.Size[i] = 1
    }
    return dsu
}

func (dsu *DSU) Find(node int) int {
    if dsu.Parent[node] != node {
        dsu.Parent[node] = dsu.Find(dsu.Parent[node])
    }
    return dsu.Parent[node]
}

func (dsu *DSU) Union(u, v int) bool {
    pu := dsu.Find(u)
    pv := dsu.Find(v)
    if pu == pv {
        return false
    }
    if dsu.Size[pu] >= dsu.Size[pv] {
        dsu.Size[pu] += dsu.Size[pv]
        dsu.Parent[pv] = pu
    } else {
        dsu.Size[pv] += dsu.Size[pu]
        dsu.Parent[pu] = pv
    }
    return true
}

func (dsu *DSU) Connected(u, v int) bool {
    return dsu.Find(u) == dsu.Find(v)
}

func solve(board [][]byte) {
    rows, cols := len(board), len(board[0])
    directions := [][]int{{1, 0}, {-1, 0}, {0, 1}, {0, -1}}
    dsu := NewDSU(rows * cols)

    for r := 0; r < rows; r++ {
        for c := 0; c < cols; c++ {
            if board[r][c] != 'O' {
                continue
            }
            if r == 0 || c == 0 || r == rows-1 || c == cols-1 {
                dsu.Union(rows*cols, r*cols+c)
            } else {
                for _, dir := range directions {
                    nr, nc := r+dir[0], c+dir[1]
                    if nr >= 0 && nr < rows && nc >= 0 &&
                       nc < cols && board[nr][nc] == 'O' {
                        dsu.Union(r*cols+c, nr*cols+nc)
                    }
                }
            }
        }
    }

    for r := 0; r < rows; r++ {
        for c := 0; c < cols; c++ {
            if !dsu.Connected(rows*cols, r*cols+c) {
                board[r][c] = 'X'
            }
        }
    }
}

class DSU(n: Int) {
    val parent: IntArray = IntArray(n + 1) { it }
    val size: IntArray = IntArray(n + 1) { 1 }

    fun find(node: Int): Int {
        if (parent[node] != node) {
            parent[node] = find(parent[node])
        }
        return parent[node]
    }

    fun union(u: Int, v: Int): Boolean {
        val pu = find(u)
        val pv = find(v)
        if (pu == pv) return false
        if (size[pu] >= size[pv]) {
            size[pu] += size[pv]
            parent[pv] = pu
        } else {
            size[pv] += size[pu]
            parent[pu] = pv
        }
        return true
    }

    fun connected(u: Int, v: Int): Boolean {
        return find(u) == find(v)
    }
}

class Solution {
    fun solve(board: Array<CharArray>) {
        val rows = board.size
        val cols = board[0].size
        val directions = arrayOf(intArrayOf(1, 0),
                                 intArrayOf(-1, 0),
                                 intArrayOf(0, 1),
                                 intArrayOf(0, -1))
        val dsu = DSU(rows * cols)

        for (r in 0 until rows) {
            for (c in 0 until cols) {
                if (board[r][c] != 'O') continue
                if (r == 0 || c == 0 || r == rows - 1 || c == cols - 1) {
                    dsu.union(rows * cols, r * cols + c)
                } else {
                    for (dir in directions) {
                        val nr = r + dir[0]
                        val nc = c + dir[1]
                        if (nr in 0 until rows &&
                            nc in 0 until cols && board[nr][nc] == 'O') {
                            dsu.union(r * cols + c, nr * cols + nc)
                        }
                    }
                }
            }
        }

        for (r in 0 until rows) {
            for (c in 0 until cols) {
                if (!dsu.connected(rows * cols, r * cols + c)) {
                    board[r][c] = 'X'
                }
            }
        }
    }
}

class DSU {
    private var parent: [Int]
    private var size: [Int]

    init(_ n: Int) {
        parent = Array(0...n)
        size = Array(repeating: 1, count: n + 1)
    }

    func find(_ node: Int) -> Int {
        if parent[node] != node {
            parent[node] = find(parent[node])
        }
        return parent[node]
    }

    func union(_ u: Int, _ v: Int) {
        let pu = find(u)
        let pv = find(v)
        if pu == pv { return }
        if size[pu] >= size[pv] {
            size[pu] += size[pv]
            parent[pv] = pu
        } else {
            size[pv] += size[pu]
            parent[pu] = pv
        }
    }

    func connected(_ u: Int, _ v: Int) -> Bool {
        return find(u) == find(v)
    }
}

class Solution {
    func solve(_ board: inout [[Character]]) {
        let ROWS = board.count
        let COLS = board[0].count
        let directions = [(1, 0), (-1, 0), (0, 1), (0, -1)]
        let dsu = DSU(ROWS * COLS + 1)
        let N = ROWS * COLS

        for r in 0..<ROWS {
            for c in 0..<COLS {
                if board[r][c] != "O" {
                    continue
                }
                if r == 0 || c == 0 || r == ROWS - 1 || c == COLS - 1 {
                    dsu.union(N, r * COLS + c)
                } else {
                    for dir in directions {
                        let nr = r + dir.0
                        let nc = c + dir.1
                        if nr >= 0, nr < ROWS, nc >= 0, nc < COLS, board[nr][nc] == "O" {
                            dsu.union(r * COLS + c, nr * COLS + nc)
                        }
                    }
                }
            }
        }

        for r in 0..<ROWS {
            for c in 0..<COLS {
                if board[r][c] == "O", !dsu.connected(N, r * COLS + c) {
                    board[r][c] = "X"
                }
            }
        }
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(m * n)$

Where $m$ is the number of rows and $n$ is the number of columns of the $board$ .