661. Image Smoother

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:

1. The value in the given matrix is in the range of [0, 255].
2. The length and width of the given matrix are in the range of [1, 150].

Approach #1: Bit. [Java]

class Solution {
    public int[][] imageSmoother(int[][] M) {
        int m = M.length, n = M[0].length;
        if (m == 0 || n == 0) return null;
        int[][] dirs = {{0, 1}, {0, -1}, {1, 0}, {-1, 0},
                        {1, 1}, {1, -1}, {-1, 1}, {-1, -1}};
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                int sum = M[i][j], cnt = 1;
                for (int k = 0; k < 8; ++k) {
                    int x = i + dirs[k][0];
                    int y = j + dirs[k][1];
                    if (x < 0 || y < 0 || x >= m || y >= n) continue;
                    sum += (M[x][y] & 0xFF);
                    cnt++;
                }
                M[i][j] |= ((sum / cnt) << 8);
            }
        }
        
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                M[i][j] >>= 8;
            }
        }
        
        return M;
    }
}

　　

Analysis:

The board has init [0-255], hence only 8-bit is used. we can use the middle 8-bit to store the new state (average value). Replace the old state to new state by shifting all value 8-bit to the right. 

Reference:

https://leetcode.com/problems/image-smoother/discuss/106593/C%2B%2B-O(1)-space-using-%22game-of-life%22-idea