Java program to check whether a string is a Palindrome

A string is said to be a palindrome if it is the same if we start reading it from left to right or right to left. So let us consider a string “str”, now the task is just to find out with its reverse string is the same as it is. 

Example:

Input : str = "abba" 
Output: Yes

Input : str = "geeks"
Output: No  

Naive Approach: By Reversing the given string and Comparing
We can check if the given string is a palindrome by comparing the original string with its reversed version. 
Below is the implementation of the above approach:

Java

/*package whatever //do not write package name here */

import java.io.*;

class GFG {
    public static boolean isPalindrome(String str)
    {
        // Initializing an empty string to store the reverse
        // of the original str
        String rev = "";

        // Initializing a new boolean variable for the
        // answer
        boolean ans = false;

        for (int i = str.length() - 1; i >= 0; i--) {
            rev = rev + str.charAt(i);
        }

        // Checking if both the strings are equal
        if (str.equals(rev)) {
            ans = true;
        }
        return ans;
    }
    public static void main(String[] args)
    {
        // Input string
        String str = "geeks";

        // Convert the string to lowercase
        str = str.toLowerCase();
        boolean A = isPalindrome(str);
        System.out.println(A);
    }
}

Output

false

In the above example, if we write ABba in place of abba, then also we should get the output as yes.  So, we must change the case of the string to either lowercase or uppercase before we check it for a palindrome. If we do not do this, we will get unexpected results. This is because the compiler checks the characters based on their ASCII value and the ASCII value of A is not the same as a. 

Approach: Our approach will be that we will first convert the string to lowercase. Then, we will take two pointers i pointing to the start of the string and j pointing to the end of the string. Keep incrementing i and decrementing j while i < j and at every step check whether the characters at these pointers are the same or not. If not then the string is not a palindrome else it is.

Example 1:

Java

// Java program to check whether a
// string is a Palindrome
// Using two pointing variables

// Main class
public class GFG {

    // Method
    // Returning true if string is palindrome
    static boolean isPalindrome(String str)
    {

        // Pointers pointing to the beginning
        // and the end of the string
        int i = 0, j = str.length() - 1;

        // While there are characters to compare
        while (i < j) {

            // If there is a mismatch
            if (str.charAt(i) != str.charAt(j))
                return false;

            // Increment first pointer and
            // decrement the other
            i++;
            j--;
        }

        // Given string is a palindrome
        return true;
    }

    // Method 2
    // main driver method
    public static void main(String[] args)
    {
        // Input string
        String str = "geeks";

        //Convert the string to lowercase
        str = str.toLowerCase();
        // passing bool function till holding true
        if (isPalindrome(str))

            // It is a palindrome
            System.out.print("Yes");
        else

            // Not a palindrome
            System.out.print("No");
    }
}

Output

No

Example 2:

Java

// Java Program to check Whether the String is Palindrome
// or Not

// Main class
class GFG {

    // Method 1
    // Returns true if string is a palindrome
    static boolean isPalindrome(String str)
    {

        // Pointers pointing to the beginning
        // and the end of the string
        int i = 0, j = str.length() - 1;

        // While there are characters to compare
        while (i < j) {

            // If there is a mismatch
            if (str.charAt(i) != str.charAt(j))
                return false;

            // Increment first pointer and
            // decrement the other
            i++;
            j--;
        }

        // Given string is a palindrome
        return true;
    }

    // Main driver method
    public static void main(String[] args)
    {
        String str = "geeks";
        String str2 = "RACEcar";

        //Change strings to lowercase
        str = str.toLowerCase();
        str2 = str2.toLowerCase();

        // For string 1
        System.out.print("String 1 :");

        if (isPalindrome(str))
            System.out.print("It is a palindrome");
        else
            System.out.print("It is not a palindrome");

        // new line for better readability
        System.out.println();

        // For string 2
        System.out.print("String 2 :");
        if (isPalindrome(str2))
            System.out.print("It is a palindrome");
        else
            System.out.print("It is not a palindrome");
    }
}

Output

String 1 :It is not a palindrome
String 2 :It is a palindrome

Recursive approach: The approach is very simple. Just like the two-pointer approach, we will check the first and the last value of the string but this time it will be through recursion.

1. We will take two pointers i pointing to the start of the string and j pointing to the end of the string. 
2. Keep incrementing i and decrementing j while i < j and at every step 
3. Check whether the characters at these pointers are the same or not. We are doing this through recursion – (i+1, j-1
4. If all the characters are the same on the ith and jth index till i>=j condition satisfies, print true else false

Below is the implementation of the above approach:

Java

//// Java program to check whether a
// string is a Palindrome using recursion

import java.io.*;

class GFG {
    public static boolean isPalindrome(int i, int j,
                                       String A)
    {
        // comparing the two pointers
        if (i >= j) {
            return true;
        }

        // comparing the characters on those pointers
        if (A.charAt(i) != A.charAt(j)) {
            return false;
        }

        // checking everything again recursively
        return isPalindrome(i + 1, j - 1, A);
    }

    public static boolean isPalindrome(String A)
    {
        return isPalindrome(0, A.length() - 1, A);
    }

    public static void main(String[] args)
    {
        // Input string
        String A = "geeks";

        // Convert the string to lowercase
        A = A.toLowerCase();
        boolean str = isPalindrome(A);
        System.out.println(str);
    }
}

Output

false