LeetCode Image Smoother

原题链接在这里：https://leetcode.com/problems/image-smoother/description/

题目：

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].

题解：

看周围8个点加自己是否是boundary内，若在就更新sum, count++. 最后计算floor average.

Time Complexity: O(m*n), m = M.length, n = M[0].length.

Space: O(m*n), res size.

AC Java:

class Solution {
    public int[][] imageSmoother(int[][] M) {
        if(M == null || M.length == 0 || M[0].length == 0){
            return M;
        }
        
        int m = M.length;
        int n = M[0].length;
        int [][] res = new int[m][n];
        
        for(int i = 0; i<m; i++){
            for(int j = 0; j<n; j++){
                int count = 0;
                int sum = 0;
                for(int iInc : new int[]{-1,0,1}){
                    for(int jInc : new int[]{-1,0,1}){
                        if(isValid(i+iInc, j+jInc, m, n)){
                            count++;
                            sum += M[i+iInc][j+jInc];
                        }
                    }
                }
                res[i][j] = sum/count;
            }
        }
        return res;
    }
    
    private boolean isValid(int i, int j, int m, int n){
        return i>=0 && i<m && j>=0 && j<n;
    }
}