Learn to check if a given string is palindrome string with simple java programs using stack, queue or simple loops. In simplest words, a string is palindrome if it is equal to it’s reverse string.
A palindrome is a word, phrase, number, or other sequence of units that may be read the same way in either direction, generally if used comma, separators or other word dividers are ignored.
1. Algorithm
1.1. First approach
To check palindrome, we can pick the characters (one by one) from start and end of string and start comparing to each other.
- Pick first character and last character of string and compare. If both matches – continue. Else string is not palindrome.
- Pick second character from start and last, compare both. If both matches – continue. Else string is not palindrome.
- Continue above comparisons till both characters to compare are same or consecutive to each other.
1.2. Second approach
Rather than comparing chars from start and end, we can also find the reverse string of the given string and compare both strings. If both strings are same, they are palindrome.
- Get character array from given string
- Build a string by iterating the array from end to beginning index
- Optionally – remove comma, separators or other word dividers from both strings
- Compare both strings
In this tutorial, we will see the examples of both approaches.
2. Check palindrome using reverse comparison
This method uses the first approach given above.
import java.util.Scanner;
public class Main
{
public static void main(String[] args) {
System.out.print("Enter any string : ");
Scanner in = new Scanner(System.in);
//Original string
String origString = in.nextLine();
int length = origString.length();
boolean isPalindrome = true;
for(int beginIndex = 0; beginIndex < length; beginIndex++)
{
if(origString.charAt(beginIndex) != origString.charAt(length-1-beginIndex)) {
System.out.println("String is not a palindrome.");
isPalindrome = false;
break;
}
}
if(isPalindrome) {
System.out.println("String is a palindrome.");
}
}
}
Enter any string : howtodoinjava String is not a palindrome. Enter any string : madam String is a palindrome.
3. Check palindrome using StringBuilder.reverse()
StringBuilder.reverse() method is shortest way to reverse a string using library functions.
import java.util.Scanner;
public class Main
{
public static void main(String[] args) {
System.out.print("Enter any string : ");
Scanner in = new Scanner(System.in);
//Original string
String origString = in.nextLine();
//Reverse string
String reverseString = new StringBuilder(origString).reverse().toString();
// Check palindrome string
if (origString.equals(reverseString)) {
System.out.println("String is a palindrome.");
} else {
System.out.println("String is not a palindrome.");
}
}
}
Enter any string : howtodoinjava String is not a palindrome. Enter any string : madam String is a palindrome.
4. Check palindrome string using java.util.Stack
Using stack’s push() and pop() methods, we can build a reverse string for a given string. Then we compare both strings.
import java.util.Scanner;
import java.util.Stack;
public class Main
{
public static void main(String[] args)
{
System.out.print("Enter any string : ");
Scanner in = new Scanner(System.in);
String origString = in.nextLine();
Stack<Character> stack = new Stack<>();
//Push all chars in stack
for (int i = 0; i < origString.length(); i++) {
stack.push(origString.charAt(i));
}
String reverseString = "";
//Pop all chars from stack one by one and build reverse string
while (!stack.isEmpty()) {
reverseString = reverseString + stack.pop();
}
//Check palindrome string
if (origString.equals(reverseString)) {
System.out.println("String is a palindrome.");
} else {
System.out.println("String is not a palindrome.");
}
}
}
Enter any string : howtodoinjava String is not a palindrome. Enter any string : racecar String is a palindrome.
5. Check palindrome string using java.util.Queue
Using Queue’s add() and remove() methods, we can build a reverse string for a given string. Then we compare both strings.
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;
public class Main
{
public static void main(String[] args)
{
System.out.print("Enter any string : ");
Scanner in = new Scanner(System.in);
String origString = in.nextLine();
Queue<Character> queue = new LinkedList<>();
for (int i = origString.length()-1; i >=0; i--) {
queue.add(origString.charAt(i));
}
String reverseString = "";
//Pop all chars from stack one by one and build reverse string
while (!queue.isEmpty()) {
reverseString = reverseString + queue.remove();
}
//Check palindrome string
if (origString.equals(reverseString)) {
System.out.println("String is a palindrome.");
} else {
System.out.println("String is not a palindrome.");
}
}
}
Enter any string : howtodoinjava String is not a palindrome. Enter any string : racecar String is a palindrome.
6. Check palindrome string using loops
This is simplest approach which simply iterated the char array backwards and creates the string by appending chars to produce reverse string.
import java.util.Scanner;
public class Main
{
public static void main(String[] args)
{
System.out.print("Enter any string : ");
Scanner in = new Scanner(System.in);
String origString = in.nextLine();
String reverseString = "";
char[] characters = origString.toCharArray();
for( int i = characters.length - 1 ; i >= 0 ; i-- ) {
reverseString = reverseString + characters[i];
}
//Check palindrome string
if (origString.equals(reverseString)) {
System.out.println("String is a palindrome.");
} else {
System.out.println("String is not a palindrome.");
}
}
}
Enter any string : water String is not a palindrome. Enter any string : madam String is a palindrome.
Drop me your questions related to
Happy Learning !!