Image Smoother

Problem

Given a 2D integer matrix M representing the gray scale of an image, you need to design a smoother to make the gray scale of each cell becomes the average gray scale (rounding down) of all the 8 surrounding cells and itself. If a cell has less than 8 surrounding cells, then use as many as you can.

Example 1:
Input:
[[1,1,1],
 [1,0,1],
 [1,1,1]]
Output:
[[0, 0, 0],
 [0, 0, 0],
 [0, 0, 0]]
Explanation:
For the point (0,0), (0,2), (2,0), (2,2): floor(3/4) = floor(0.75) = 0
For the point (0,1), (1,0), (1,2), (2,1): floor(5/6) = floor(0.83333333) = 0
For the point (1,1): floor(8/9) = floor(0.88888889) = 0

Solution

public class Solution {
    public int[][] imageSmoother(int[][] matrix) {
        if (matrix == null || matrix.length == 0) return new int[0][0];
        int m = matrix.length, n = matrix[0].length;
        int[][] res = new int[m][n];
        int[][] moves = new int[][] {{-1, 0}, {1, 0}, {0, -1}, {0, 1}, {-1, -1}, {1, 1}, {1, -1}, {-1, 1}};
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                int sum = matrix[i][j];
                int count = 1;
                for (int[] move : moves) {
                    int I = i + move[0];
                    int J = j + move[1];
                    if (I >= 0 && I < m && J >= 0 && J < n) {
                        count++;
                        sum += matrix[I][J];
                    }
                }
                res[i][j] = sum / count;
            }
        }
        return res;
    }
}

Analysis

Typical matrix counting problem
Since we are gonna return a new int[][] we don't have to care about the in-place changes
Just visit the surrounding 8 (at most) neighbors and record what we need