public class Solution {

    int[][] dirs = {{-1,0},{1,0},{0,-1},{0,1}};
    int R;
    int C;

    public void solve(char[][] board) {
        R = board.length;
        C = R==0?0:board[0].length;
        for(int r = 0; r<R; ++r) {
            if(board[r][0]=='O') {
                fill(board, r, 0);
            }
            if(board[r][C-1]=='O') {
                fill(board, r, C-1);
            }
        }
        for(int c=0; c<C; ++c) {
            if(board[0][c]=='O') {
                fill(board, 0, c);
            }
            if(board[R-1][c]=='O') {
                fill(board, R-1, c);
            }
        }

        for(int r = 0; r<R; ++r) {
            for(int c = 0; c<C; ++c) {
                if(board[r][c]=='O') {
                    board[r][c]='X';
                } else if(board[r][c]=='1') {
                    board[r][c]='O';
                }
            }
        }
    }

    void fill(char[][]board, int x, int y) {
        board[x][y] = '1';
        for(int[] dir:dirs) {
            if(0<=x+dir[0] && x+dir[0]<R && 0<=y+dir[1] && y+dir[1]<C && board[x+dir[0]][y+dir[1]]=='O') {
                fill(board, x+dir[0], y+dir[1]);
            }
        }
    }
}