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

if (board == null || board.length == 0) return;

int rows = board.length;

int cols = board[0].length;

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

if (board[i][0] == 'O') dfs(i,1,board);

if (board[i][cols-1] =='O') dfs(i,cols-2,board);

}

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

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

dfs(1,i,board);

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

dfs(rows-2,i,board);

}

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

for (int j = 1; j < cols-1; j++) {

if (board[i][j] == '*') board[i][j] ='O';

else if(board[i][j] =='O') board[i][j] ='X';

}

}

}

private void dfs(int i, int j, char[][] board) {

if (i<=0 || j<=0 || i>= board.length-1 || j>=board[0].length-1 || board[i][j] =='X') return;

if (board[i][j] == '*') return;

if (board[i][j] =='O') board[i][j] ='*';

dfs(i+1,j,board);

dfs(i-1,j,board);

dfs(i,j+1,board);

dfs(i,j-1,board);

}