ISC Practicals and Theory - Palindrome Number (Recursive Technique)

A Palindrome Number reads the same from left to right and from right to left. In this version the reverse of the number is found by recursive technique: the method reverse calls itself, each call removing one digit.

Definition

A number is a Palindrome Number if it is equal to the number formed by writing its digits in reverse order.

• $12321$ reversed is $12321$, so it is a palindrome.
• $1230$ reversed is $321$ (the leading zero is lost), so it is not a palindrome.

A recursive method is a method that calls itself, and it must have a base case that stops the calls.

Algorithm

1. main creates the object ob and calls accept() to read num, then check().
2. check() prints Invalid input if num is negative; otherwise it calls reverse(num, 0) and compares the value returned with num.
3. reverse(int n, int rev) works as follows:
   • Base case: if n == 0, all digits are used, so return rev.
   • Recursive case: call reverse(n / 10, rev * 10 + n % 10). The last digit of n is added to rev and the last digit is removed from n.
4. If the value returned equals num, it is a Palindrome Number, otherwise it is not.

Program

reverse(int n, int rev) is the recursive method; check() starts it with rev = 0.

import java.util.Scanner;
public class PalindromeRec
{
    int num;
    PalindromeRec()
    {
        num = 0;
    }
    void accept()
    {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        num = sc.nextInt();
    }
    int reverse(int n, int rev)
    {
        if (n == 0)
            return rev;
        return reverse(n / 10, rev * 10 + n % 10);
    }
    void check()
    {
        if (num < 0)
            System.out.println("Invalid input");
        else if (reverse(num, 0) == num)
            System.out.println(num + " is a Palindrome Number");
        else
            System.out.println(num + " is not a Palindrome Number");
    }
    public static void main(String args[])
    {
        PalindromeRec ob = new PalindromeRec();
        ob.accept();
        ob.check();
    }
}

Variable description table

Sample input and output

Enter a number: 12321
12321 is a Palindrome Number

Enter a number: 1230
1230 is not a Palindrome Number

Enter a number: -5
Invalid input

Dry run

Input: num = 1230. The first call is reverse(1230, 0).

The value 321 comes back through all the calls. It is not equal to 1230, so the output is 1230 is not a Palindrome Number, the same as the real run above.

Variations

1. Range "Accept two numbers m and n and display all the Palindrome Numbers between m and n."
2. Palindrome Word "Check whether a word is a palindrome using a recursive method."