NearestMaze

Given a 2d array as a maze with '. (open space)' and 'G (guard)', update each open spaces with the distance to its nearest guard.
Example -> 
matrix : 
.  .  .  G .
.  .  G .  .
.  .  .  . G
answer -> 
2  1  1  G  1
2  1  G  1  1
2  1  1  1  G


public char[][] distMaze(char[][] maze) {
  final int M = maze.length;
  if (M == 0) return maze;
  final int N = maze[0].length;
  if (N == 0) return maze;
    
  Queue <Point> q = new LinkedList<Point>();
  for (int y = 0; y < M; y++) {
    for (int x = 0; x < N; x++) {
      if (maze[y][x] == 'G') {
        q.add(new Point(y, x, 0));
      }
    }
  }
    
  if (q.size() == 0) {
  
    for (int y = 0; y < M; y++) {
      for (int x = 0; x < N; x++) {
        maze[y][x] = '-';
      }
    }
        
  } else {
    
    while(!q.isEmpty()) {
      Point p = q.remove();
      relax2(maze, y - 1, x - 1, p, q);
      relax2(maze, y - 1, x    , p, q);
      relax2(maze, y - 1, x + 1, p, q);
      relax2(maze, y    , x - 1, p, q);
      relax2(maze, y    , x + 1, p, q);
      relax2(maze, y + 1, x - 1, p, q);
      relax2(maze, y + 1, x    , p, q);
      relax2(maze, y + 1, x + 1, p, q);
    }
    
  }
  return maze;
}

public void relax2(char[][] maze, int y, int x, Point p, Queue<Point> q) {
  if (relax(maze, y, x, p)) {
    q.add(new Point(y, x, maze[y][x] - '0');
  }
}

// return true if update maze[y][x]
public boolean relax(char[][] maze, int y, int x, Point p) {
  final int M = maze.length;
  final int N = maze[0].length;
  if (y < 0 || y >= M || x < 0 || x >= N || maze[y][x] != '.') {
    return false;
  }
  maze[y][x] = '0' + p.dist + 1;
  return true;
}

class Point {
  int y;
  int x;
  int dist;
  public Point(int y, int x, int dist) {
    this.y = y;
    this.x = x;
    this.dist = dist;
  }
}