2373. Largest Local Values in a Matrix

Problem Link

Description

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You are given an n x n integer matrix grid.

Generate an integer matrix maxLocal of size (n - 2) x (n - 2) such that:

• maxLocal[i][j] is equal to the largest value of the 3 x 3 matrix in grid centered around row i + 1 and column j + 1.

In other words, we want to find the largest value in every contiguous 3 x 3 matrix in grid.

Return the generated matrix.

 

Example 1:

Input: grid = [[9,9,8,1],[5,6,2,6],[8,2,6,4],[6,2,2,2]]
Output: [[9,9],[8,6]]
Explanation: The diagram above shows the original matrix and the generated matrix.
Notice that each value in the generated matrix corresponds to the largest value of a contiguous 3 x 3 matrix in grid.

Example 2:

Input: grid = [[1,1,1,1,1],[1,1,1,1,1],[1,1,2,1,1],[1,1,1,1,1],[1,1,1,1,1]]
Output: [[2,2,2],[2,2,2],[2,2,2]]
Explanation: Notice that the 2 is contained within every contiguous 3 x 3 matrix in grid.

 

Constraints:

• n == grid.length == grid[i].length
• 3 <= n <= 100
• 1 <= grid[i][j] <= 100

Solution

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Python3

class Solution:
    def largestLocal(self, grid: List[List[int]]) -> List[List[int]]:
        rows, cols = len(grid), len(grid[0])
        res = [[None] * (cols - 2) for _ in range(rows - 2)]
 
        for i in range(rows - 2):
            for j in range(cols - 2):
                currMax = -inf
 
                for i2 in range(i, i + 3):
                    for j2 in range(j, j + 3):
                        currMax = max(currMax, grid[i2][j2])
                
                res[i][j] = currMax
 
        return res