1314. Matrix Block Sum

Given a m x n matrix mat and an integer k, return a matrix answer where each answer[i][j] is the sum of all elements mat[r][c] for:

• i - k <= r <= i + k,
• j - k <= c <= j + k, and
• (r, c) is a valid position in the matrix.

class Solution {
    int getOrZero(vector<vector<int>>& mat, int n, int m, int y, int x) {
        if(0 <= x and x < m and 0 <= y and y < n)
            return mat[y][x];
        return 0;
    }
public:
    vector<vector<int>> matrixBlockSum(vector<vector<int>>& mat, int k) {
        int n = mat.size(), m = mat[0].size();
        vector<vector<int>> res(n,vector<int>(m));
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                mat[i][j] += getOrZero(mat,n,m,i-1,j) + getOrZero(mat,n,m,i,j-1) -getOrZero(mat,n,m,i-1,j-1);
            }
        }
        
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                res[i][j] = getOrZero(mat,n,m, min(i + k, n-1), min(j + k, m-1))
                    - getOrZero(mat,n,m,min(i+k,n-1),j-k-1)
                    - getOrZero(mat,n,m,i-k-1,min(j+k,m-1))
                    + getOrZero(mat,n,m,i-k-1,j-k-1);
            }
        }
        return res;
    }
};