Repaint.java

import java.util.*;

public class Repaint
{

    static final int INF = 1000000010; // even
    static int[] dx = {1, 0, -1, 0, 1, -1, -1, 1};
    static int[] dy = {0, 1, 0, -1, 1, 1, -1, -1};

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        int H = sc.nextInt();
        int W = sc.nextInt();

        char[][] a = new char[H][W];

        for (int i = 0; i < H; i++) {
            a[i] = sc.next().toCharArray();
        }

        char[][] b = new char[H][W];
        for (int i = 0; i < H; i++)
            Arrays.fill(b[i], '.');

        // Perform ONE repaint
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {

                if (a[i][j] == '#') {

                    for (int d = 0; d < 8; d++) {

                        int ni = i + dx[d];
                        int nj = j + dy[d];

                        if (0 <= ni && ni < H && 0 <= nj && nj < W
                                && a[ni][nj] == '.') {

                            b[ni][nj] = '#';
                        }
                    }
                }
            }
        }

        int[][] dist = new int[H][W];
        for (int i = 0; i < H; i++)
            Arrays.fill(dist[i], INF);

        Queue<int[]> q = new ArrayDeque<>();

        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                if (b[i][j] == '#') {
                    dist[i][j] = 0;
                    q.offer(new int[]{i, j});
                }
            }
        }

        while (!q.isEmpty()) {

            int[] cur = q.poll();
            int x = cur[0];
            int y = cur[1];

            for (int d = 0; d < 8; d++) {

                int nx = x + dx[d];
                int ny = y + dy[d];

                if (0 <= nx && nx < H && 0 <= ny && ny < W
                        && dist[nx][ny] == INF) {

                    dist[nx][ny] = dist[x][y] + 1;
                    q.offer(new int[]{nx, ny});
                }
            }
        }

        StringBuilder sb = new StringBuilder();

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

            sb.setLength(0);

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

                if (dist[i][j] % 2 == 0)
                    sb.append('.');
                else
                    sb.append('#');
            }

            System.out.println(sb);
        }

        sc.close();
    }
}