Print a Matrix Diagonally

Algorithm/Insights

rowCount = number of rows
columnCount = number of columns
Then, number of diagonals will be = rowCount + columnCount - 1 as depicted in the diagram below.


Step 1: Print first rowCount diagonals
Print diagonals that start from the first column
elements.


Step 2: Print next columnCount - 1 diagonals
Print diagonals that start from the last row
elements.


Step 1 details: Print first rowCount diagonals
Iterate to print diagonals from row k = 0 to rowCount - 1.
1: Start with row = k and col = 0
2: Print the element matrix[row][col]
3: Decrement row by 1, increment col by 1 till row >= 0 and  col < columnCount

Step 2 details: Print next columnCount - 1 diagonals
Iterate to print diagonals from column k = 1 to columnCount - 1
1: Start with last row, row = rowCount - 1 and col = k
2: Print the element matrix[row][col]
3: Decrement row by 1, increment col by 1 till row >= 0 and  col < columnCount

Code Snippet

			
package com.ideserve.questions.saurabh;

import java.util.Arrays;

/**
* <b>IDeserve <br>
* <a href="https://www.youtube.com/c/IDeserve">https://www.youtube.com/c/IDeserve</a>
* Given a matrix of mxn dimensions, print the elements of the matrix
* in diagonal order.
*
* @author Saurabh
*/
public class PrintMatrixDiagonally {

    public static void printMatrixDiagonally(int[][] matrix) {

        int row, col;
        int rowCount = matrix.length;
        int columnCount = matrix[0].length;

        for (int k = 0; k < rowCount; k++) {
            for (row = k, col = 0; row >= 0 && col < columnCount; row--, col++) {
                System.out.print(matrix[row][col] + " ");
            }
            System.out.println();
        }

        for (int k = 1; k < columnCount; k++) {
            for (row = rowCount - 1, col = k; row >= 0 && col < columnCount; row--, col++) {
                System.out.print(matrix[row][col] + " ");
            }
            System.out.println();
        }
    }

    public static void main(String[] args) {
        int row = 4, col = 5;
        int k = 1;
        int matrix[][] = new int[row][col];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                matrix[i][j] = k++;
            }
        }

        System.out.println("Input Matrix");
        for (int i = 0; i < matrix.length; i++) {
            System.out.println(Arrays.toString(matrix[i]));
        }
        
        System.out.println("\nPrinting Matrix Diagonally");
        printMatrixDiagonally(matrix);
    }

}
		

Order of the Algorithm

Time Complexity is O(mn)
Space Complexity is O(1)