Array reverse in Java – Algorithms for interview

There is an array {1, 2, 3, 4} and it needs to be expanded (or inverted) to get an array {4, 3, 2, 1}.

We find the middle of the array - array.length / 2. Iterate over the elements of the array from the beginning to the middle and simultaneously swap the elements with index i and array.length - i - 1. To exchange values, we use the option with the introduction of a temporary variable, discussed in the previous section.

public class ArrayInverter {
    public static void invert(int[] array) {
        for (int i = 0; i < array.length / 2; i++) {
            int tmp = array[i];
            array[i] = array[array.length - i - 1];
            array[array.length - i - 1] =  tmp;
        }
    }
}

Please note that there is no main() method in the ArrayInverter class – we will call the invert() method from another class. To call a method located in another class, write the class name – ArrayInverter.invert(array1):

import java.util.Arrays;

public class ArrayInverterTest1 {
    public static void main(String[] args) {
        int[] array1 = new int[]{};
        System.out.print(Arrays.toString(array1) + " => ");
        ArrayInverter.invert(array1);
        System.out.println(Arrays.toString(array1));

        array1 = new int[]{0};
        System.out.print(Arrays.toString(array1) + " => ");
        ArrayInverter.invert(array1);
        System.out.println(Arrays.toString(array1));

        array1 = new int[]{0, 1};
        System.out.print(Arrays.toString(array1) + " => ");
        ArrayInverter.invert(array1);
        System.out.println(Arrays.toString(array1));

        array1 = new int[]{0, 1, 2};
        System.out.print(Arrays.toString(array1) + " => ");
        ArrayInverter.invert(array1);
        System.out.println(Arrays.toString(array1));

        array1 = new int[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        System.out.print(Arrays.toString(array1) + " => ");
        ArrayInverter.invert(array1);
        System.out.println(Arrays.toString(array1));
    }
}

If we look closely at the code in the ArrayInverterTest1 class, we can see that one block of code is repeated five times. Duplicate code is a bad practice, so let’s put the duplicate code in a separate method that will be called five times with different values:

import java.util.Arrays;

public class ArrayInverterTest2 {
    public static void main(String[] args) {
        testInvert(new int[]{});
        testInvert(new int[]{0});
        testInvert(new int[]{0, 1});
        testInvert(new int[]{0, 1, 2, 3, 4});
    }

    private static void testInvert(int[] array) {
        System.out.print(Arrays.toString(array) + " => ");
        ArrayInverter.invert(array);
        System.out.println(Arrays.toString(array));
    }
}