[leetcode-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].

思路：

实现类似卷积的效果。

    vector<vector<int>> imageSmoother(vector<vector<int>>& M)
{
    vector<vector<int>> ret( M );

    int sum = 0;
    int rowNum = M.size();
    int colNum = M[0].size();
    int cnt = 0;
    for( int i = 0; i < rowNum; i++ )
    {
        for( int j = 0; j < colNum; j++ )
        {
            sum = 0;
            cnt = 0;
            for( int k = i-1; k <= i+1; k++ )
            {
                for( int p = j - 1; p <= j+1; p++ )
                {
                    if( k < 0 || p < 0 || k >= rowNum || p >= colNum )
                    {
                        continue;
                    }
                    sum += M[k][p];
                    cnt++;
                }
            }
            ret[i][j] = sum / cnt;
        }
    }

    return ret;
}