661. Image Smoother

Q:

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].

A:

class Solution {
public:
    vector<vector<int>> imageSmoother(vector<vector<int>>& M) {
        int m = M.size(), n = M[0].size();
        vector<vector<int> > res(m, vector<int>(n,0));
        for(int i =0;i<m;++i)
            for(int j =0;j<n;++j)
            {
                int c = 0, sum =0;
                for(int a=-1;a<=1;++a)
                    for(int b = -1;b<=1;++b)
                    {
                        if(i+a>=0 && i+a<m && j+b >=0 && j+b <n){
                            ++c;
                            sum += M[i+a][j+b];
                        }
                    }
                res[i][j] = sum/c;
            }
        return res;        
    }
};