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); } } |
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" ); } } |
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" ); } } |
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.
- 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. We are doing this through recursion – (i+1, j-1
- 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); } } |
false
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