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Gonzalo Diaz
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[Hacker Rank] Interview Preparation Kit: Array: 2D Array - DS. Solved ✅.
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‎algorithm-exercises-java/src/main/java/ae/hackerrank/interview_preparation_kit/arrays/TwoDarray.java

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package ae.hackerrank.interview_preparation_kit.arrays;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.stream.Collectors;
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/**
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* 2D Array - DS.
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*
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* @link Problem definition [[docs/hackerrank/interview_preparation_kit/arrays/2d_array.md]]
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*/
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public class TwoDarray {
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private TwoDarray() { }
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private static List<Integer> getHourGlass(List<List<Integer>> arr, int positionX, int positionY) {
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List<Integer> result = new ArrayList<>();
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// top
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result.add(arr.get(positionX - 1).get(positionY - 1));
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result.add(arr.get(positionX - 1).get(positionY));
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result.add(arr.get(positionX - 1).get(positionY + 1));
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// middle
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result.add(arr.get(positionX).get(positionY));
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// bottom
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result.add(arr.get(positionX + 1).get(positionY - 1));
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result.add(arr.get(positionX + 1).get(positionY));
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result.add(arr.get(positionX + 1).get(positionY + 1));
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return result;
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}
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/**
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* hourglassSum.
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*/
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public static Integer hourglassSum(List<List<Integer>> arr) {
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int matrixSize = 0;
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if (arr == null) {
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return null;
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}
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matrixSize = arr.size();
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int matrixStartIndex = 1;
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int matrixEndIndex = matrixSize - 2;
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Integer maxHourGlassSum = null;
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for (int i = matrixStartIndex; i <= matrixEndIndex; i++) {
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for (int j = matrixStartIndex; j <= matrixEndIndex; j++) {
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List<Integer> currentHourGlass = getHourGlass(arr, i, j);
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int hourGlassSum = currentHourGlass.stream()
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.collect(Collectors.summingInt(Integer::intValue));
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if (maxHourGlassSum == null || hourGlassSum > maxHourGlassSum) {
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maxHourGlassSum = hourGlassSum;
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}
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}
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}
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return maxHourGlassSum;
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}
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}

‎algorithm-exercises-java/src/test/java/ae/hackerrank/interview_preparation_kit/arrays/TwoDarrayTest.java

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package ae.hackerrank.interview_preparation_kit.arrays;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import java.io.IOException;
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import java.util.ArrayList;
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import java.util.List;
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import org.junit.jupiter.api.BeforeAll;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.api.TestInstance;
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import org.junit.jupiter.api.TestInstance.Lifecycle;
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import util.JsonLoader;
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@TestInstance(Lifecycle.PER_CLASS)
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class TwoDarrayTest {
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public static class TwoDarrayTestCase {
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public String title;
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public List<List<Integer>> input;
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public long expected;
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}
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private List<TwoDarrayTestCase> testCases;
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@BeforeAll
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public void setup() throws IOException {
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String path = String.join("/", "hackerrank",
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"interview_preparation_kit",
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"arrays",
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"2d_array.testcases.json");
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this.testCases = JsonLoader.loadJson(path, TwoDarrayTestCase.class);
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}
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@Test void testHourglassSum() {
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for (TwoDarrayTestCase testCase : testCases) {
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long solutionFound = TwoDarray.hourglassSum(testCase.input);
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assertEquals(testCase.expected, solutionFound,
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"%s(%s) answer must be: %s".formatted(
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"TwoDarray.hourglassSum",
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testCase.input.toString(),
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testCase.expected
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)
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);
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}
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}
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@Test void testHourglassSumEdgeCases() {
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List<List<Integer>> input = null;
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Integer expected = null;
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Integer solutionFound = TwoDarray.hourglassSum(null);
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assertEquals(expected, solutionFound,
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"%s(%s) answer must be: %s".formatted(
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"TwoDarray.hourglassSum",
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input,
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expected
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)
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);
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input = new ArrayList<List<Integer>>();
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expected = null;
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solutionFound = TwoDarray.hourglassSum(null);
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assertEquals(expected, solutionFound,
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"%s(%s) answer must be: %s".formatted(
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"TwoDarray.hourglassSum",
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input,
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expected
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)
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);
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}
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}
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‎algorithm-exercises-java/src/test/resources/hackerrank/interview_preparation_kit/arrays/2d_array.testcases.json

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[
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{
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"title": "Sample Test Case 0",
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"input": [
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[1, 1, 1, 0, 0, 0],
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[0, 1, 0, 0, 0, 0],
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[1, 1, 1, 0, 0, 0],
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[0, 0, 2, 4, 4, 0],
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[0, 0, 0, 2, 0, 0],
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[0, 0, 1, 2, 4, 0]
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],
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"expected": 19
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}
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]

‎docs/hackerrank/interview_preparation_kit/arrays/2d_array.md

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# [Arrays: 2D Array - DS](https://www.hackerrank.com/challenges/2d-array)
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- Difficulty: ` #easy `
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- Category: ` #ProblemSolvingBasic `
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Given a 6 × 6 2D Array, `arr`:
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```text
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1 1 1 0 0 0
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0 1 0 0 0 0
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1 1 1 0 0 0
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0 0 0 0 0 0
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0 0 0 0 0 0
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0 0 0 0 0 0
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```
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An hourglass in `A` is a subset of values with indices falling in this pattern
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in `arr`'s graphical representation:
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```text
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a b c
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d
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e f g
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```
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There are `16` hourglasses in `arr`.
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An hourglass sum is the sum of an hourglass' values.
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Calculate the hourglass sum for every hourglass in `arr`,
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then print the maximum hourglass sum. The array will always be 6 × 6.
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## Example
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arr =
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```text
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-9 -9 -9 1 1 1
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0 -9 0 4 3 2
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-9 -9 -9 1 2 3
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0 0 8 6 6 0
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0 0 0 -2 0 0
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0 0 1 2 4 0
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```
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The `16` hourglass sums are:
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```text
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-63, -34, -9, 12,
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-10, 0, 28, 23,
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-27, -11, -2, 10,
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9, 17, 25, 18
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```
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The highest hourglass sum is `26` from the hourglass beginning
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at row `1`, column `2`:
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```text
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0 4 3
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1
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8 6 6
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````
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**Note**: If you have already solved the Java domain's Java 2D Array challenge,
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you may wish to skip this challenge.
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## Function Description
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Complete the function hourglassSum in the editor below.
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hourglassSum has the following parameter(s):
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- `int arr[6][6]`: an array of integers
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## Returns
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- int: the maximum hourglass sum
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## Input Format
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Each of the `6` lines of inputs `arr[i]` contains space-separated integers `arr[i][j]`.
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## Constraints
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- $9 \leq arr[i][j] \leq 9$
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- $0 \leq i, j \leq 5$
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## Output Format
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Print the largest (maximum) hourglass sum found in `arr`.
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## Sample Input
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```text
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1 1 1 0 0 0
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0 1 0 0 0 0
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1 1 1 0 0 0
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0 0 2 4 4 0
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0 0 0 2 0 0
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0 0 1 2 4 0
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```
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## Sample Output
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```text
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19
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```
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## Explanation
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`arr` contains the following hourglasses:
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```text
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111 110 100 000
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1 0 0 0
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111 110 100 000
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010 100 000 000
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0 1 0 0
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002 024 244 440
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111 110 100 000
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0 2 4 4
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000 002 020 200
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002 024 244 440
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0 0 2 0
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001 012 124 240
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```
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The hourglass with the maximum sum (`19`) is:
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```text
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2 4 4
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2
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1 2 4
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```

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