# [LeetCode]#2325. Decode the Message

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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

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