2373

2373. Largest Local Values in a Matrix (Easy)

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

Companies: Google

Related Topics:
Array, Matrix

Solution 1.

// OJ: https://leetcode.com/problems/largest-local-values-in-a-matrix
// Author: github.com/lzl124631x
// Time: O(MN)
// Space: O(1) extra space
class Solution {
public:
    vector<vector<int>> largestLocal(vector<vector<int>>& A) {
        int M = A.size(), N = A[0].size();
        vector<vector<int>> ans(M - 2, vector<int>(N - 2));
        for (int i = 1; i < M - 1; ++i) {
            for (int j = 1; j < N - 1; ++j) {
                int mx = 0;
                for (int dx = -1; dx <= 1; ++dx) {
                    for (int dy = -1; dy <= 1; ++dy) {
                        mx = max(mx, A[i + dx][j + dy]);
                    }
                }
                ans[i - 1][j - 1] = mx;
            }
        }
        return ans;
    }
};