LeetCode-Image Smoother

Description:

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

Note:

The value in the given matrix is in the range of [0, 255].

The length and width of the given matrix are in the range of [1, 150].

class Solution
    public int[][] imageSmoother(int[][] M) {
        int[][] result = new int[M.length][M[0].length];
        for(int i=0; i<result.length; i++) {
            for(int j=0; j<result[i].length; j++) {
                result[i][j] = imageCellSmoother(M, i, j);
            }
        }
        return result;
    }

    private int imageCellSmoother(int[][] M, int r, int c) {
        int sum = 0;
        int cnt = 0;
        int leftTopR = r - 1 >= 0 ? r - 1 : r;
        int leftTopC = c - 1 >= 0 ? c - 1 : c;
        int rightBottomR = r + 1 < M.length ? r + 1 : r;
        int rightBottomC = c + 1 < M[0].length ? c + 1 : c;
        for(int i=leftTopR; i<=rightBottomR; i++) {
            for(int j=leftTopC; j<=rightBottomC; j++) {
                sum += M[i][j];
                cnt++;
            }
        }
        return sum