<p>Given a <code>m x n</code> matrix <code>mat</code> and an integer <code>k</code>, return <em>a matrix</em> <code>answer</code> <em>where each</em> <code>answer[i][j]</code> <em>is the sum of all elements</em> <code>mat[r][c]</code> <em>for</em>:</p> <ul> <li><code>i - k <= r <= i + k,</code></li> <li><code>j - k <= c <= j + k</code>, and</li> <li><code>(r, c)</code> is a valid position in the matrix.</li> </ul> <p> </p> <p><strong>Example 1:</strong></p> <pre> <strong>Input:</strong> mat = [[1,2,3],[4,5,6],[7,8,9]], k = 1 <strong>Output:</strong> [[12,21,16],[27,45,33],[24,39,28]] </pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> mat = [[1,2,3],[4,5,6],[7,8,9]], k = 2 <strong>Output:</strong> [[45,45,45],[45,45,45],[45,45,45]] </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>m == mat.length</code></li> <li><code>n == mat[i].length</code></li> <li><code>1 <= m, n, k <= 100</code></li> <li><code>1 <= mat[i][j] <= 100</code></li> </ul>