Java: Create an array of its anti-diagonals from a given square matrix

Java Array: Exercise-37 with Solution

Write a Java program to create an array of its anti-diagonals from a given square matrix.

Example:
Input :
1 2
3 4
Output:
[
[1],
[2, 3],
[4]
]
Input: 
[10, 20, 30]
[50, 60, 70]
[90, 100, 110]
Output:
[10]
[20, 50]
[30, 60, 90]
[70, 100]
[110]

Sample Solution:

Java Code:

//https://github.com/nagajyothi/Arrays/blob/master/Diagonal.java
// Import necessary Java libraries.
import java.util.*;

// Define a class named Exercise37.
public class Exercise37 {
    // A method to compute and return diagonal elements efficiently.
    public static ArrayList<ArrayList<Integer>> diagonalEfficient(ArrayList<ArrayList<Integer>> A) {
        // Create a list of lists to store the diagonal elements.
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        // Get the number of rows and columns in the 2D array.
        int m = A.size();
        int n = A.get(0).size();
        // Create a list to temporarily store diagonal elements.
        ArrayList<Integer> temp = new ArrayList<Integer>();
        // Add the first diagonal element to the result.
        temp.add(A.get(0).get(0));
        result.add(new ArrayList<Integer>(temp));
        
        // Iterate through the columns of the first row and add diagonal elements to the result.
        int i = 0;
        int j = i + 1;
        while (j < n) {
            int k = i;
            int l = j;
            temp.clear();
            while (k < m && l >= 0) {
                temp.add(A.get(k).get(l));
                k++;
                l--;
            }
            result.add(new ArrayList<Integer>(temp));
            j++;
        }
        
        // Iterate through the rows of the last column and add diagonal elements to the result.
        i = 1;
        j = n - 1;
        while (i < m) {
            int k = i;
            int l = j;
            temp.clear();
            while (k < m && l >= 0) {
                temp.add(A.get(k).get(l));
                k++;
                l--;
            }
            result.add(new ArrayList<Integer>(temp));
            i++;
        }
        
        // Clear the temporary list and return the result.
        temp.clear();
        return result;
    }

    // The main method for executing the program.
    public static void main(String[] args) {
        // Create a 2D array represented as a list of lists.
        ArrayList<ArrayList<Integer>> A = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> temp = new ArrayList<Integer>();
        temp.add(10);
        temp.add(20);
        temp.add(30);
        A.add(new ArrayList<Integer>(temp));
        temp.clear();
        
        temp.add(50);
        temp.add(60);
        temp.add(70);
        A.add(new ArrayList<Integer>(temp));
        temp.clear();
        
        temp.add(90);
        temp.add(100);
        temp.add(110);
        A.add(new ArrayList<Integer>(temp));
        temp.clear();
        
        // Print the original 2D array.
        for (ArrayList<Integer> t : A) {
            System.out.println(t);
        }
        
        // Call the diagonalEfficient method and print the result.
        ArrayList<ArrayList<Integer>> result = diagonalEfficient(A);
        for (ArrayList<Integer> t : result) {
            System.out.println(t);
        }
    }
}

Sample Output:

                                                                             
[10, 20, 30]
[50, 60, 70]
[90, 100, 110]
[10]
[20, 50]
[30, 60, 90]
[70, 100]
[110]

Flowchart:

Java Code Editor:

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