JAVA: How do I determine if the input is a palindrome?

Question

Please use the method below.

I am trying to get rid of all white spaces, punctuations and make everything lowercase. Then I want to see whether the string is a palindrome (same when read from the front and back.

I can't figure it out.

public static void main(String[] args) {


        String word=null;
        String reverse="";
        Scanner console = new Scanner(System.in);

        System.out.print("Please enter a word or a phrase:");
        word = console.nextLine();

        word=word.replaceAll("\\s+",""); //removes white space
        word=word.replaceAll("[^a-zA-Z ]", ""); //removes all punctuation
        word=word.toLowerCase();


        for(int i=word.length()-1; i>=0; i--) {
        reverse +=word.charAt(i);


        }

        for(int i=0; i<word.length(); i++) {
            System.out.print(word);
        if(word.charAt(i) != reverse.charAt(i)) {
            System.out.println("Not a Palindrome");
        }else {
                System.out.println("Palindrome");```

That is just a minor thing, it's the actual code used to determine the palindrome you should look at and that question has several answers that are worthwhile to study. Or are you only looking for a copy/paste solution? — Joakim Danielson, Feb 16 '20 at 21:36

score 0 · Answer 1 · answered Feb 16 '20 at 21:00

You can have two indices i and j, where i starts at 0-index and j starts at the end of array. Do i++ and j-- and at each iteration check if the element is same and when j>i you break the loop and say its palindrome or else it is not Palindrome.

You can do this iteratively or recursively.

Hope it helps!

score 0 · Answer 2 · answered Feb 16 '20 at 21:19

First, you can combine all of your regular expressions into one (first make the input lowercase, then eliminate everything not a lowercase letter). Second, you can eliminate building a temporary String to check if it's a palindrome; because you can iterate the characters in the word and check the front with the back. Like,

System.out.print("Please enter a word or a phrase:");
String word = console.nextLine().toLowerCase().replaceAll("[^a-z]", "");
boolean palindrome = true;
for (int i = 0; i < word.length() / 2; i++) {
    if (word.charAt(i) != word.charAt(word.length() - i - 1)) {
        palindrome = false;
        break;
    }
}
if (!palindrome) {
    System.out.println("Not a Palindrome");
} else {
    System.out.println("Palindrome");
}

If you decide you want to reverse word anyway for a less efficient implementation. Use StringBuilder, because it can be done in one line (and I think it looks more readable).

System.out.print("Please enter a word or a phrase:");
String word = console.nextLine().toLowerCase().replaceAll("[^a-z]", "");
if (!new StringBuilder(word).reverse().toString().equals(word)) {
    System.out.println("Not a Palindrome");
} else {
    System.out.println("Palindrome");
}

JAVA: How do I determine if the input is a palindrome?

2 Answers2