[Java - 2D Array] #Java #2DArray #tutorial #interview #Array

0 - Introduction to 2D Array.md

Similar to a one-dimensional array, a two-dimensional array also consists of a sequence of elements. But the elements can be laid out in a rectangular grid rather than a line.

An Example

// "static void main" must be defined in a public class.
public class Main {
    private static void printArray(int[][] a) {
        for (int i = 0; i < a.length; ++i) {
            System.out.println(a[i]);
        }
        for (int i = 0; i < a.length; ++i) {
            for (int j = 0; a[i] != null && j < a[i].length; ++j) {
                System.out.print(a[i][j] + " ");
            }
            System.out.println();
        }
    }
    public static void main(String[] args) {
        System.out.println("Example I:");
        int[][] a = new int[2][5];
        printArray(a);
        System.out.println("Example II:");
        int[][] b = new int[2][];
        printArray(b);
        System.out.println("Example III:");
        b[0] = new int[3];
        b[1] = new int[5];
        printArray(b);
    }
}

Principle

In some languages, the multidimensional array is actually implemented internally as a one-dimensional array while in some other languages, there is actually no multidimensional array at all.

1. C++

stores the two-dimensional array as a one-dimensional array.

The picture below shows the actual structure of a M * N array A: So actually A[i][j] equals to A[i * N + j] if we defined A as a one-dimensional array which also contains M * N elements.

2. In Java

the two-dimensional array is actually a one-dimensional array which contains M elements, each of which is an array of N integers.

The picture below shows the actual structure of a two-dimensional array A in Java:

Dynamic 2D Array

Similar to the one-dimensional dynamic array, we can also define a dynamic two-dimensional array. Actually, it can be just a nested dynamic array. You can try it out by yourself.

1 - Diagonal Traverse.md

Given a matrix of M x N elements (M rows, N columns), return all elements of the matrix in diagonal order as shown in the below image.

Example:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output:  [1,2,4,7,5,3,6,8,9]

Explanation:

Note:

1. The total number of elements of the given matrix will not exceed 10,000.