How To Reverse a Number In Java? Algorithm & Program

How To Reverse a Number In Java? Algorithm & Program

In this blog, we are going to see the Algorithm and different ways to Reverse a number in java. Basically reversing the number is to inverting the number program. The Program for reversing the number can be used to check if a number is a palindrome number or not. If on reversing a number, we get the same number, then it’s a palindrome number; otherwise, not.

Algorithm :

1. Input:  num
2. Initialize rev_num = 0
3. Loop while num > 0
   1. Multiply rev_num by 10 and add remainder of num 
4. divide by 10 to rev_num
   1. rev_num = rev_num*10 + num%10;
   1. Divide num by 10
5. Return rev_num

Ways to Reverse a number in java .

• Using a while loop.
• Using for a loop.
• Using recursion.

Reverse a number in java using a while loop (Without user interaction)

This is the program when the number already initialized. In this Program, you just need to change the value of the variable num as per your requirement.

public class ReverseNumber {
    public static void main(String[] args) {
        int num = 1234, reversed = 0;
        while(num != 0) {
            int digit = num % 10;
            reversed = reversed * 10 + digit;
            num /= 10;
        }
        System.out.println("Reversed Number: " + reversed);
    }
}

Reverse a number in java using a while loop (User interaction)

The program will prompt a user to input the number and then it will reverse the same number using a while loop.

import java.util.Scanner;
class ReverseNumberWhile
{
   public static void main(String args[])
   {
      int num=0;
      int reversenum =0;
      System.out.println("Input your number and press enter: ");
      Scanner in = new Scanner(System.in);
      num = in.nextInt();
      while( num != 0 )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of input number is: "+reversenum);
   }
}

Reverse a number in java using a for loop (Without user interaction)

This is the program when the number already initialized. In this Program, you just need to change the value of the variable num as per your requirement.

public class ReverseNumber {
    public static void main(String[] args) {
        int num = 1234567, reversed = 0;
        for(;num != 0; num /= 10) {
            int digit = num % 10;
            reversed = reversed * 10 + digit;
        }
        System.out.println("Reversed Number: " + reversed);
    }
}

Reverse a number in java using a for loop (User interaction)

The program will prompt a user to input the number and then it will reverse the same number using a for a loop.

import java.util.Scanner;
class ForLoopReverseDemo
{
   public static void main(String args[])
   {
      int num=0;
      int reversenum =0;
      System.out.println("Input your number and press enter: ");
      Scanner in = new Scanner(System.in);
      num = in.nextInt();
      for( ;num != 0; )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of specified number is: "+reversenum);
   }
}

Reverse a number in java using recursion (Without user interaction)

This is the program when the number already initialized. In this Program, you just need to change the value of the variable num as per your requirement.

class ReverseNumberDemo{ 
   public static void main(String args[])
   {
      int num=123456789;
      int reversenum =0;
      while( num != 0 )
      {
          reversenum = reversenum * 10;
          reversenum = reversenum + num%10;
          num = num/10;
      }

      System.out.println("Reverse of specified number is: "+reversenum);
   }
}

Reverse a number in java using recursion (User interaction)

The program will prompt a user to input the number and then it will reverse the same number using recursion.

import java.util.Scanner;
class RecursionReverseDemo
{
   public static void reverseMethod(int number) {
       if (number < 10) {
	   System.out.println(number);
	   return;
       }
       else {
           System.out.print(number % 10);
           reverseMethod(number/10);
       }
   }
   public static void main(String args[])
   {
	int num=0;
	System.out.println("Input your number and press enter: ");
	Scanner in = new Scanner(System.in);
	num = in.nextInt();
	System.out.print("Reverse of the input number is:");
	reverseMethod(num);
	System.out.println();
   }
}

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