Diagonal Difference in Java | Find difference between sums of two diagonals | HackerRank problem

Java Solution for HackerRank Diagonal Difference problem 

Problem Description :

Given a square matrix, calculate the absolute difference between the sums of its diagonals. 

For example, the square matrix arr is shown below: 

1 2 3
4 5 6
9 8 9

The left to right diagonal = 1 + 5 + 9 = 15 The right to left diagonal = 3 + 5 + 9 = 17 Their absolute difference is | 15 - 17 | = 2.

See full description :

• Diagonal Difference HackerRank Problem Description 
  

Sample Input :

Row and Column Size : 3

2 4 6
1 3 5
7 8 -9

Sample Output : 2

Explanation : left to right diagonal = 2 + 3 - 9 = -4. The right to left diagonal = 6 + 3 + 7 = 16. So the absolute difference is | -4 - 16 = 20 |

Lest see solution and its explanation.

Solution 1

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.function.*;
import java.util.regex.*;
import java.util.stream.*;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.toList;

class Result {

    public static int diagonalDifference(List<List<Integer>> arr) {
   
        int leftDiagonal = 0;
        int rightDiagonal = 0;
	    
        for (int i = 0; i < arr.size(); i++) {
	    leftDiagonal += arr.get(i).get(i);
	    rightDiagonal += arr.get(i).get((arr.get(i).size() - 1) - i);
	}
        
        return Math.abs(leftTotal - rightTotal);

    }

}

public class Solution {
    public static void main(String[] args) throws IOException {
        BufferedReader bufferedReader = new BufferedReader(
            new InputStreamReader(System.in));
        BufferedWriter bufferedWriter = new BufferedWriter(
            new FileWriter(System.getenv("OUTPUT_PATH")));

        int n = Integer.parseInt(bufferedReader.readLine().trim());

        List<List<Integer>> arr = new ArrayList<>();

        IntStream.range(0, n).forEach(i -> {
            try {
                arr.add(
                    Stream.of(bufferedReader.readLine().replaceAll("\\s+$", "")
                        .split(" "))
                        .map(Integer::parseInt)
                        .collect(toList())
                );
            } catch (IOException ex) {
                throw new RuntimeException(ex);
            }
        });

        int result = Result.diagonalDifference(arr);

        bufferedWriter.write(String.valueOf(result));
        bufferedWriter.newLine();

        bufferedReader.close();
        bufferedWriter.close();
    }
}

Code explanation :

• We already have Integer of 2D List. We just have to add logic in given method.
• In this problem, we just need to return absolute (Positive value) Sum of left to right diagonal - Sum of right to left diagonal.
• Define two integer value leftDiagonal, rightDiagonal and initialize with 0 value. 
• Traverse through given 2D list from 0 to list size and store sum of left to right diagonal value to leftDiagonal variable and right to left diagonal value to rightDiagonal variable. 
• In loop using 3 * 3 matrix, we are getting value of leftDiagonal through following index : 0 + 4 + 8, and right Diagonal through following index : 2 + 4 + 6.
  
• list.get(0).get(0) means 0th row and 0th column.
  
• Last return absolute value of leftDiagonal - rightDiagonal value.

Output explanation :

List = 1st row [1, 2, 3],
           2nd row [4, 5, 6],
           3rd row [9, 8, 9]

• i = 0, leftDiagonal = 0, rightDiagonal = 0, list size = 3
• leftDiagonal = 0 + arr.get(0).get(0) = [0, 0]
  leftDiagonal  =  0 + 1 = 1
• rightDiagonal = 0 + arr.get(0).get((arr.get(0).size() - 1) - 0) = [0, 2]
  rightDiagonal = 0 + 3 = 3
  
  
• i = 1, leftDiagonal = 1, rightDiagonal = 3
• leftDiagonal = 1 + arr.get(1).get(1) = [1, 1]
  leftDiagonal  =  1 + 5 = 6
• rightDiagonal = 3 + arr.get(1).get((arr.get(1).size() - 1) - 1) = [1, 1]
  rightDiagonal = 3 + 5 = 8
  
  
• i = 2, leftDiagonal = 6, rightDiagonal = 8
• leftDiagonal = 6 + arr.get(2).get(2) = [2, 2]
  leftDiagonal  =  6 + 9 = 15
  
• rightDiagonal = 8 + arr.get(2).get((arr.get(2).size() - 1) - 2) = [2, 0]
  rightDiagonal = 8 + 9 = 17
  
  
• leftDiagonal = 15, rightDiagonal = 17
• Math.abs(15 - 17) = 2
  

We can directly take left and right diagonal value one by one through loop and return answer.

Solution 2

public static int diagonalDifference(List<List<Integer>> arr) {
        
        int difference = 0;
        for (int i = 0; i < arr.size(); i++) {
            difference += arr.get(i).get(i) - arr.get(i).get((arr.get(i).size()-1)-i);
        }
        
        return Math.abs(difference);
}

Lets see another solution using java 8

Solution 3

int leftDiagonal = arr.stream()
                .map(i -> i.get(arr.indexOf(i)))
                .collect(Collectors.toList())
                .stream()
                .reduce(0, (a, b) -> a+b);

int rightDiagonal = arr.stream()
                .map(i -> i.get((arr.size() - 1) - arr.indexOf(i)))
                .collect(Collectors.toList())
                .stream()
                .reduce(0, (a, b) -> a+b);

return Math.abs(leftDiagonal - rightDiagonal);

Learn more about Java 8 Stream API map(), collect() and reduce() methods.

1. collect() method in Java 8 
2. map() method in Java 8
   
3. reduce() method in Java 8 
   

Happy Coding.

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