1. Problem statement

Given an array A of integers, reverse this array A in linear running time using constant memory.

2. Solution

The steps to reverse an array can be shown with the help of following figure:

3. Program

Following is the program to reverse the array.

import java.util.Arrays;

public class ReverseArray {
    public int[] reverse(int[] nums) {
        // initialize low and high index
        int lowIndex = 0;
        int highIndex = nums.length - 1;

        // while lowIndex is less than high index
        while (lowIndex < highIndex) {
            // swap low index and high index
            swap(nums, lowIndex, highIndex);

            //increment low index and decrement high index
            lowIndex++;
            highIndex--;
        }

        return nums;
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

    public static void main(String[] args){
        int[] nums = {1,2,3,4,5};

        ReverseArray reverseArray = new ReverseArray();
        int[] reversedArray = reverseArray.reverse(nums);

        System.out.println(Arrays.toString(reversedArray));
    }
}

Output:

[5, 4, 3, 2, 1]

Approach 2

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class ReverseArray {
    public static void main(String[] args) {
        Integer[] numbers = {1, 2, 3, 4, 5}; // Example array

        List<Integer> reversedList = Arrays.stream(numbers) // Convert to stream
                .collect(Collectors.toList()); // Convert to list

        Collections.reverse(reversedList); // Reverse the list

        Integer[] reversedArray = reversedList.toArray(new Integer[0]); // Convert back to array

        System.out.println("Reversed Array: " + Arrays.toString(reversedArray));
    }
}