[LeetCode]#2325. Decode the Message

Environment: Python 3.8

Key technique: chain

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.

Analysis:

1. Calculate output matrix size
2. Use mask conception to find the local largest number.

Solution:

class Solution:
    def largestLocal(self, grid):
 
        n = len(grid)-2
        ans = [[0]*n for _ in range(n)]
        
        for i in range(n):
            for j in range(n):
                ans[i][j] = max(chain(grid[i  ][j:j+3], 
                                      grid[i+1][j:j+3],
                                      grid[i+2][j:j+3]))
        return  ans

Submissions:

Reference:

The chain function is very good and it let me know python deeply. Thanks below reference.

https://leetcode.com/problems/largest-local-values-in-a-matrix/discuss/2425602/Python3-oror-6-lines-chain-oror-TM%3A-145-ms14.2-MB