2373 - Largest Local Values in a Matrix (Easy)

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.

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.

class Solution:
    def largestLocal(self, grid: List[List[int]]) -> List[List[int]]:
        n = len(grid)
        # the result matrix is always (n - 2) * (n - 2)
        res = [[0] * (n - 2) for _ in range(n - 2)]
        # iterate all possible 3 x 3 grids
        for i in range(n - 2):
            for j in range(n - 2):
                # for (i, j) being at top-left,
                # iterate to check the max in this 3 x 3 grid
                for ii in range(i, i + 3):
                    for jj in range(j, j + 3):
                        res[i][j] = max(res[i][j], grid[ii][jj])
        return res

object Solution:
  def largestLocal(grid: Array[Array[Int]]): Array[Array[Int]] =
    Array.tabulate(grid.size - 2, grid.size - 2): (i, j) =>
      grid.slice(i, i + 3).map(_.slice(j, j + 3).max).max