How To Reverse A Number In Java

In this article, we will see how to reverse a number in Java.

Let’s dive in.

Problem Statement

Reversing a number means swapping the digits. To Reverse a number we convert a number from its original form to a form where its digits are in the opposite order.

For Example:

Input:

96724

Output:

42769

Solution

We will see three different ways to reverse a number in Java. Those are:

1. Using Loops
2. Using Recursion
3. Using StringBuilder

// Java Program To Reverse A Number Using Loops

public class ReverseNumber {
    public static void main(String[] args) {
        int num = 12345; // Replace this with the number you want to reverse
        int rev = 0;

        while (num != 0) {
            int digit = num % 10; // Extract the last digit
            rev = rev * 10 + digit; // Add the digit to the reversed number
            num /= 10; // Remove the last digit
        }

        System.out.println("Reversed Number: " + rev);
    }
}

Reversed Number: 54321

Code Explanation

At the end of the program, rev contains the reversed number, which is 54321.

// Java Program To Reverse A Number Using Recursion

public class ReverseNumberRecursion {
    public static int reverseRecursive(int num, int rev) {
        // Base case: If num is 0, return the reversed number (rev)
        if (num == 0) {
            return rev;
        }

        // Extract the last digit of num
        int lastDigit = num % 10;

        // Build 'rev' by multiplying it by 10 and adding the last digit
        rev = rev * 10 + lastDigit;

        // Proceed to the next digit by dividing 'num' by 10
        num /= 10;

        // Make a recursive call with the updated 'num' and 'rev'
        return reverseRecursive(num, rev);
    }

    public static void main(String[] args) {
        int num = 12345; // Replace this with the number you want to reverse
        int reversedNum = reverseRecursive(num, 0);

        System.out.println("Reversed Number: " + reversedNum);
    }
}

Reversed Number: 54321

Code Explanation

The final reversed number is 54321, as shown in step 8.

// Java Program To Reverse A Number Using StringBuilder

public class ReverseNumberWithStringBuilder {

    public static void main(String[] args) {

        int num = 12345; // Replace this with the number you want to reverse

        // Step 1: Convert the number to a string
        String numString = Integer.toString(num);

        // Step 2: Create a StringBuilder object to store the reversed string
        StringBuilder reversedBuilder = new StringBuilder();

        // Step 3 and Step 4: Iterate through the characters from right to left and append to StringBuilder
        for (int i = numString.length() - 1; i >= 0; i--) {
            reversedBuilder.append(numString.charAt(i));
        }

        // Step 5: Convert the StringBuilder back to an integer to get the reversed number
        int reversedNum = Integer.parseInt(reversedBuilder.toString());

        System.out.println("Reversed Number: " + reversedNum);
    }
}

Reversed Number: 54321

Code Explanation

The final reversed number is 54321, as shown in step 11.

Space and Time Complexity

Note: In the context of these complexities, n represents the number of digits in num, which is equivalent to log10(num).

Key Takeaways

• Basic approach to reverse a number on extracting digits systematically from the number and reconstructing the reversed number from right to left.
• Recursive approach also follows similar digit extraction logic, but makes a recursive call repeatedly instead of a loop. Base case is reached when number becomes 0.
• Converting the number to a String allows leveraging StringBuilder to simply append characters in reverse order. StringBuilder can then be converted back to number.

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