Chapter 16. PC #5. Palindrome Detector (page 1073) A palindrome is any word, phr

Question

Chapter 16. PC #5. Palindrome Detector (page 1073)

A palindrome is any word, phrase, or sentence that reads the same forward and backward. Here are some well-known palindromes:

Able was I, ere I saw Elba

A man, a plan, a canal, Panama

Desserts, I stressed

Kayak

Write a (JAVA) boolean method that uses RECURSION to determine whether a String argument is a palindrome. The method should return true if the argument reads the same forward and backward. Demonstrate the method in a program.

The program should ask the user to enter a string, which is checked for palindrome property. The program displays whether the given input is a palindrome or not, then prompts the user to enter another string. If the user enters QUIT (case insensitive, then exit the program).

Explanation / Answer

One approach to check palindrome using recursion is to check first and last character for a match, if it matches then pass the substring with first and last character removed to the same function recursively, until we get length of string as 0 or 1. If at any point in between first and last character does'nt match return false.

We have to clean the input string at first, that is remove white spaces, commas and dots and convert entire string to lower case. For that I have used replaceAll() and toLowerCase() .

Below is the entire program:

/* package codechef; // don't place package name! */

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

class palindrome
{
    static boolean checkPalindrome(String str){
      
        if(str.length()==0||str.length()==1)
            return true;
        if(str.charAt(0)==str.charAt(str.length()-1))
           return checkPalindrome(str.substring(1,str.length()-1));
          
    return false;
    }
   public static void main (String[] args) throws java.lang.Exception
   {
       Scanner sc=new Scanner(System.in);
      
       while(true){
            String input=sc.nextLine();
            //if quit encountered come out of loop
            if(input.equalsIgnoreCase("quit"))
                break;
      
          //remove all spaces, commas and dots
          String cleanString = input.replaceAll("[, . \s]", "").toLowerCase();
            if(checkPalindrome(cleanString))
                System.out.println(input+" is palindrome");
            else
                System.out.println(input+" is NOT palindrome");
       }
   }
}

Sample Input:

Able was I, ere I saw Elba
A man, a plan, a canal, Panama
Desserts, I stressed
Kayak
sdf
QUIT

Sample output:

Able was I, ere I saw Elba is palindrome
A man, a plan, a canal, Panama is palindrome
Desserts, I stressed is palindrome
Kayak is palindrome
sdf is NOT palindrome

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