LeetCode-in-Java

1314. Matrix Block Sum

Medium

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.

Example 1:

Input: mat = [[1,2,3],[4,5,6],[7,8,9]], k = 1

Output: [[12,21,16],[27,45,33],[24,39,28]]

Example 2:

Input: mat = [[1,2,3],[4,5,6],[7,8,9]], k = 2

Output: [[45,45,45],[45,45,45],[45,45,45]]

Constraints:

• m == mat.length
• n == mat[i].length
• 1 <= m, n, k <= 100
• 1 <= mat[i][j] <= 100

Solution

public class Solution {
    public int[][] matrixBlockSum(int[][] mat, int k) {
        int rows = mat.length;
        int cols = mat[0].length;
        int[][] prefixSum = new int[rows + 1][cols + 1];
        for (int i = 1; i <= rows; i++) {
            for (int j = 1; j <= cols; j++) {
                prefixSum[i][j] =
                        mat[i - 1][j - 1]
                                - prefixSum[i - 1][j - 1]
                                + prefixSum[i - 1][j]
                                + prefixSum[i][j - 1];
            }
        }

        int[][] result = new int[rows][cols];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                int iMin = Math.max(i - k, 0);
                int iMax = Math.min(i + k, rows - 1);
                int jMin = Math.max(j - k, 0);
                int jMax = Math.min(j + k, cols - 1);
                result[i][j] =
                        prefixSum[iMin][jMin]
                                + prefixSum[iMax + 1][jMax + 1]
                                - prefixSum[iMax + 1][jMin]
                                - prefixSum[iMin][jMax + 1];
            }
        }
        return result;
    }
}

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