0661. Image Smoother

https://leetcode.com/problems/image-smoother

Description

An image smoother is a filter of the size 3 x 3 that can be applied to each cell of an image by rounding down the average of the cell and the eight surrounding cells (i.e., the average of the nine cells in the blue smoother). If one or more of the surrounding cells of a cell is not present, we do not consider it in the average (i.e., the average of the four cells in the red smoother).

Given an m x n integer matrix img representing the grayscale of an image, return the image after applying the smoother on each cell of it.

Example 1:

**Input:** img = [[1,1,1],[1,0,1],[1,1,1]]
**Output:** [[0,0,0],[0,0,0],[0,0,0]]
**Explanation:**
For the points (0,0), (0,2), (2,0), (2,2): floor(3/4) = floor(0.75) = 0
For the points (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

Example 2:

**Input:** img = [[100,200,100],[200,50,200],[100,200,100]]
**Output:** [[137,141,137],[141,138,141],[137,141,137]]
**Explanation:**
For the points (0,0), (0,2), (2,0), (2,2): floor((100+200+200+50)/4) = floor(137.5) = 137
For the points (0,1), (1,0), (1,2), (2,1): floor((200+200+50+200+100+100)/6) = floor(141.666667) = 141
For the point (1,1): floor((50+200+200+200+200+100+100+100+100)/9) = floor(138.888889) = 138

Constraints:

• m == img.length

• n == img[i].length

• 1 <= m, n <= 200

• 0 <= img[i][j] <= 255

ac

class Solution {
    private final int[][] DIRS = new int[][] {{1,1},{1,0},{1,-1},{0,1},{0,0},{0,-1},{-1,1},{-1,0},{-1,-1}}; 

    public int[][] imageSmoother(int[][] M) {
        // edge cases
        if (M == null || M.length == 0 || M[0].length == 0) return new int[0][0];
        int row = M.length;
        int col = M[0].length;

        int[][] matrix = new int[row][col];
        // iterate
        for (int r = 0; r < row; r++) {
            for (int c = 0; c < col; c++) {
                int cells = 0;
                int sum = 0;
                for (int[] d : DIRS) {
                    int r2 = r + d[0];
                    int c2 = c + d[1];
                    if (r2 < 0 || r2 >= row || c2 < 0 || c2 >= col) continue; // invalid cell
                    cells++;
                    sum += M[r2][c2];
                }
                matrix[r][c] = sum / cells;
            }
        }

        return matrix;
    }
}

/*
count cells, sum
*/