Image Smoother — LeetCode 661 Python Solution

The problem

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).

Example

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

Python solution

class Solution:
    def imageSmoother(self, img: List[List[int]]) -> List[List[int]]:
        m, n = len(img), len(img[0])
        ans = [[0] * n for _ in range(m)]
        for i in range(m):
            for j in range(n):
                s = cnt = 0
                for x in range(i - 1, i + 2):
                    for y in range(j - 1, j + 2):
                        if 0 <= x < m and 0 <= y < n:
                            cnt += 1
                            s += img[x][y]
                ans[i][j] = s // cnt
        return ans

Complexity

Pattern: Matrix and Grid

Treat a 2-D grid as a graph whose neighbours are the four adjacent cells. LeetCode 661. Image Smoother is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Matrix.

The matrix and grid guide has the Python template for the pattern and the 216 LeetCode problems that use it.

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