Largest Local Values in a Matrix

Tags : leetcode, array, matrix, cpp, easy

Problem Statement - link #

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.

Examples #

Example 1:

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:

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

Solutions #

class Solution {
public:
    int helper(vector<vector<int>> &grid,int x, int y){
        int res = grid[x][y];
        for(int i=0;i<3;i++){
            for(int j=0;j<3;j++){
                res = max(res,grid[x+i][y+j]);
            }
        }
        return res;
    }
    vector<vector<int>> largestLocal(vector<vector<int>>& grid) {
        int n=grid.size();
        vector<vector<int>> ans(n-2,vector<int>(n-2));
        for(int i=1;i<n-1;i++){
            for(int j=1;j<n-1;j++){
                ans[i-1][j-1] = helper(grid,i-1,j-1);
            }
        }
        return ans;
    }
};