Algorithm and program to reverse a Number in java

🔢 Reverse a Number in Java (Without Using Built-in Reverse Methods)

🧩 What problem are we solving?

The goal is to reverse the digits of an integer — for example, converting 122 into 221.
This is a classic example to understand how to work with loops, integer division, and the modulo operator in Java.

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⚙️ Algorithm (Step by Step)

We want to rebuild the number by extracting its last digit and adding it to a new reversed number in each step.

Formula:

Reverse = (Reverse * 10) + (Number % 10)
Number  = Number / 10

Step-by-Step Example

Let’s take the input number 122:

Step	Number	Reverse Calculation	Reverse Result	New Number
1	122	(0 × 10) + (122 % 10)	2	12
2	12	(2 × 10) + (12 % 10)	22	1
3	1	(22 × 10) + (1 % 10)	221	0

✅ Final Output: 221

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🧠 Key Concept

• % (modulo) gives the last digit of a number.
  → 122 % 10 = 2

• / (integer division) removes the last digit.
  → 122 / 10 = 12

• Multiplying the current reverse by 10 shifts digits left to make room for the next digit.

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📘 Java Implementation

package com.vinod.test;

/**
 * ReverseNumberExample
 *
 * Problem:
 *   Reverse the digits of an integer without using built-in reverse functions.
 *
 * Algorithm:
 *   1. Initialize reverse = 0.
 *   2. Repeat while number != 0:
 *        - Extract the last digit using number % 10.
 *        - Multiply reverse by 10 and add the extracted digit.
 *        - Divide number by 10 to remove the last digit.
 *   3. Print the reversed number.
 *
 * Example:
 *   Input:  122
 *   Output: 221
 *
 * Time Complexity:
 *   O(log10(n)) → proportional to the number of digits.
 *
 * Space Complexity:
 *   O(1)
 *
 * Author: Vinod Kariyathungal Kumaran
 */
public class ReverseNumberExample {
    public static void main(String[] args) {
        int reverse = 0;
        int number = 122;

        System.out.println("Input number = " + number);
        System.out.println("Intermediate steps:");

        while (number != 0) {
            reverse = (reverse * 10) + (number % 10);
            System.out.println("Reverse = " + reverse);
            number = number / 10;
            System.out.println("Number = " + number);
        }

        System.out.println("Reversed number = " + reverse);

        // Alternative method using StringBuilder
        StringBuilder sb = new StringBuilder(String.valueOf(122));
        System.out.println("Reversed number using StringBuilder = " + sb.reverse().toString());
    }
} 

🧩 Output

Input number = 122
Intermediate steps:
Reverse = 2
Number = 12
Reverse = 22
Number = 1
Reverse = 221
Number = 0
Reversed number = 221
Reversed number using StringBuilder = 221