int[] dx = {1, -1, 0, 0};

int[] dy = {0, 0, 1, -1};

public void solve(char[][] board) {

int n = board.length;

if (n == 0) {

return;

}

int m = board[0].length;

Queue<int[]> queue = new LinkedList<int[]>();

for (int i = 0; i < n; i++) {

if (board[i][0] == 'O') {

queue.offer(new int[]{i, 0});

}

if (board[i][m - 1] == 'O') {

queue.offer(new int[]{i, m - 1});

}

}

for (int i = 1; i < m - 1; i++) {

if (board[0][i] == 'O') {

queue.offer(new int[]{0, i});

}

if (board[n - 1][i] == 'O') {

queue.offer(new int[]{n - 1, i});

}

}

while (!queue.isEmpty()) {

int[] cell = queue.poll();

int x = cell[0], y = cell[1];

board[x][y] = 'A';

for (int i = 0; i < 4; i++) {

int mx = x + dx[i], my = y + dy[i];

if (mx < 0 || my < 0 || mx >= n || my >= m || board[mx][my] != 'O') {

continue;

}

queue.offer(new int[]{mx, my});

}

}

for (int i = 0; i < n; i++) {

for (int j = 0; j < m; j++) {

if (board[i][j] == 'A') {

board[i][j] = 'O';

} else if (board[i][j] == 'O') {

board[i][j] = 'X';

}

}

}

}