Java Program to Reverse an Array Without Using Another Array

Introduction

Reversing an array in-place (i.e., without using an additional array) is a common problem in programming. This involves swapping elements from the start and end of the array until the middle of the array is reached. This guide will show you how to create a Java program that reverses an array in-place.

Problem Statement

Create a Java program that:

• Takes an array of integers as input.
• Reverses the array in-place without using another array.
• Returns and displays the reversed array.

Example 1:

• Input: [1, 2, 3, 4, 5]
• Output: [5, 4, 3, 2, 1]

Example 2:

• Input: [10, 20, 30, 40, 50]
• Output: [50, 40, 30, 20, 10]

Solution Steps

1. Initialize the Array: Define an array with integer values.
2. Reverse the Array In-Place:
   • Use two pointers: one starting at the beginning and the other at the end of the array.
   • Swap the elements at these two positions.
   • Move the pointers towards the center of the array until they meet or cross each other.
3. Display the Result: Print the reversed array.

Java Program

Java Program to Reverse an Array Without Using Another Array

import java.util.Arrays;

/**
 * Java Program to Reverse an Array Without Using Another Array
 * Author: https://www.javaguides.net/
 */
public class ReverseArrayInPlace {

    public static void main(String[] args) {
        // Step 1: Initialize the array
        int[] array = {1, 2, 3, 4, 5};

        // Step 2: Reverse the array in-place
        reverseArray(array);

        // Step 3: Display the reversed array
        System.out.println("Reversed array: " + Arrays.toString(array));
    }

    // Method to reverse the array in-place
    public static void reverseArray(int[] array) {
        int left = 0;
        int right = array.length - 1;

        while (left < right) {
            // Swap the elements at left and right pointers
            int temp = array[left];
            array[left] = array[right];
            array[right] = temp;

            // Move the pointers towards the center
            left++;
            right--;
        }
    }
}

Explanation

• Input: The program uses a predefined array of integers.

• Reversing the Array In-Place:

  • The reverseArray() method takes an array as input.
  • Two pointers, left and right, are initialized to point to the first and last elements of the array, respectively.
  • In a while loop, the elements at these pointers are swapped, and the pointers are moved towards the center of the array.
  • The loop continues until the left pointer is no longer less than the right pointer, indicating that the array has been fully reversed.

• Output: The program prints the reversed array using Arrays.toString() for easy formatting.

Output Example

Example 1:

Reversed array: [5, 4, 3, 2, 1]

Example 2:

Reversed array: [50, 40, 30, 20, 10]

Explanation of Output:

• Example 1: The array [1, 2, 3, 4, 5] is reversed in-place to become [5, 4, 3, 2, 1].
• Example 2: The array [10, 20, 30, 40, 50] is reversed in-place to become [50, 40, 30, 20, 10].

Conclusion

This Java program effectively reverses an array in-place without using another array. By using two pointers that start at opposite ends of the array and move towards the center, the program efficiently swaps elements to achieve the reversal. This approach is memory-efficient because it avoids the need for additional storage, making it suitable for scenarios where memory usage is a concern.