Java Program to Find Reverse of a Number Using Recursion

Recursion is a process by which a function calls itself repeatedly till it falls under the base condition and our motive is achieved.

To solve any problem using recursion, we should simply follow the below steps:

• Step 1: Assume the smaller problem from the problem that is similar to the bigger/original problem.
• Step 2: Decide the answer to the smallest valid input or smallest invalid input which would act as our base condition.
• Step 3: Approach the solution and link the answer to the smaller problem given by the recursive function to find the answer to the bigger/original problem using it.

Example:

Input:  14689
Output: 98641
Input:  7654321
Output: 1234567

Approach 1:

• In this approach, we can simply print the unit digit of a number.
• And then call the recursive function for the number after removing this unit digit (number/10)
• And this process continues till the number is reduced to a single-digit number. 

Example:

    num = 82695
        reverse(82695)
           |
           |__print(5)
              reverse(8269)
                    |
                    |__print(9)
                       reverse(826)
                            |
                            |__print(6)
                               reverse(82)
                                   |
                                   |__print(2)
                                      reverse(8)
                                          |
                                          |__print(8)
                                             return

Java

// Java program to reverse
// an integer recursively
class GFG {
  
    // Recursive function to print
    // the number in reversed form
    public static void Reverse(int num)
    {

        // base condition to end recursive calls
        if (num < 10) {
            System.out.println(num);
            return;
        }

        else {

            // print the unit digit of the given number
            System.out.print(num % 10);

            // calling function for remaining number other
            // than unit digit
            Reverse(num / 10);
        }
    }

    // driver code
    public static void main(String args[])
    {
        // number to be reversed
        int num = 98765;

        System.out.print("Reversed Number: ");

        // calling recursive function 
        // to print the number in
        // reversed form
        Reverse(num);
    }
}

Output

Reversed Number: 56789

Time complexity: O(log₁₀n) where n is given input number.

Auxiliary space: O(1)

Approach 2: 

• In this approach, we can simply maintain a variable in which we can store the number reversed till now.
• We can do this by extracting a unit digit from the number and then adding this extracted integer into the reversed number.
• But the key factor here is that we have to multiply the reversed number by 10 before adding this extracted number to the reversed number.
  

Example:

num = 48291
ans = 0       -> variable to store reversed number
        
How this works:    
    reverse(num)
          |
          |__ temp = num % 10    -> extracting unit digit from number
                ans = ans*10 + temp   -> adding temp at unit position in reversed number 
          reverse(num/10)    -> calling function for remaining number
Implementation:
          reverse(48291)
                |
                |__ temp=1
                      ans= 0*10 + 1  --> ans=1
                      reverse(4829)
                        |
                       |__ temp=9
                              ans= 1*10 + 9  --> ans=19
                                reverse(482)
                                     |
                                     |__ temp= 2
                                         ans= 19*10 +2  --> ans=192
                                         reverse(48)
                                              |
                                              |__ temp=8
                                                  ans=192*10 + 8  --> ans=1928
                                                  reverse(4)
                                                      |
                                                      |__ temp=4
                                                          ans=1928*10 +4 --> ans=19284
                                                          reverse(0)
                                                              |
                                                              |__ return ans
                          

Java

// Java program to reverse an integer recursively
class GFG {
  
    // Variable to store reversed
    // number after every
    // recursive call
    static int ans = 0;

    static int Reverse(int var)
    {

        // base condition to end the
        // recursive calling of function
        if (var == 0) {

            // We have reversed the
            // complete number and
            // stored in ans variable
            return ans;
        }

        if (var > 0) {

            // temp variable to store the digit at unit
            // place in the number
            int temp = var % 10;

            // Add this temp variable in the ans variable
            // which stores the number reversed till now
            ans = ans * 10 + temp;

            // recursive calling of function to reverse the
            // remaining number
            Reverse(var / 10);
        }

        // returning final answer when the number is
        // reversed completely
        return ans;
    }

    public static void main(String[] args)
    {

        // Number to be reversed
        int var = 98765;
      
        // Variable to store reversed number returned by
        // reverse function
        int rev;

        // Calling reverse function and storing the return
        // value in rev variable
        rev = Reverse(var);

        // Printing the Reversed Number
        System.out.println("Reversed number: " + rev);
    }
}

Output

Reversed number: 56789

Time complexity: O(logn) where n is input number to be reversed.

Auxiliary space: O(1)

Note: Java does not throw an exception when an overflow occurs so a problem of overflow might occur if the reversed number is greater than Integer.MAX_VALUE (2147483647) in method 2 but there will be no such problem in method 1.

Approach 3:

While using recursion we can look at the number of digits and using that we can concatenate the reversed numbers.

Java

import java.io.*;
import java.lang.*;

class GFG {

    public int reverseDigits(int number)
    {
        if (number < 10)
            return number;

        int remainder = number % 10;

        int reversedNumber
            = this.reverseDigits(number / 10);

        int numberOfDigits = (int)Math.log10(number) + 1;
        return remainder
            * (int)Math.pow(10, numberOfDigits - 1)
            + reversedNumber;
    }

    public static void main(String[] args)
    {
          GFG obj = new GFG();
          int reversedNumber1 = obj.reverseDigits(987);
          int reversedNumber2 = obj.reverseDigits(144321);
        System.out.println(reversedNumber1);
        System.out.println(reversedNumber2);      
    }
}

Output

789
123441