How to check palindrome ?

// Method - I:

1. public class PalindromAlgoritham {
2. 
   
3. public static void main(String[] args) {
4. String s1 = "ABCBA";
5. int p = s1.length() - 1;
6. boolean flag = true;
7. for (int i = 0; i < s1.length()/2; i++) {
8. if (s1.charAt(i) != s1.charAt(p)) {
9. flag=false;
10. break;
11. }
12. p--;
13. }
14. System.out.println((flag?"Palindrome":"Not Palindrome"));
15. }
16. 
    
17. }

Output: Palindrome
Time complexity: O(n/2)

// Method - II:

1. public class PalindromeByCharAtAlgorithm {
2. 
   
3. public static void main(String[] args) {
4. String s1 = "ABCDCBA ABCDCBA ABCDCBA";
5. String rev = "";
6. for (int i = s1.length() - 1; i >= 0; i--) {
7. rev = rev + s1.charAt(i);
8. }
9. if (s1.equals(rev)) {
10. System.out.println("Palindrome !!");
11. } else {
12. System.out.println("Not Palindrome !!");
13. }
14. 
    
15. }
16. 
    
17. }

Output: Palindrome !!
Time Complexity: O(nlog(n))

// Method - III:

1. public class PalindromeByStringBuilderAlgorithm {
2. 
   
3. public static void main(String[] args) {
4. 
   
5. String str = "313";
6. int value = Integer.parseInt(str);
7. 
   
8. StringBuilder sb = new StringBuilder(str);
9. String reverse = sb.reverse().toString();
10. int rvalue = Integer.parseInt(reverse);
11. 
    
12. if (value == rvalue) {
13. System.out.println("Palindrome !!");
14. } else {
15. System.out.println("Not palindrome !!");
16. }
17. }
18. }

Output: Palindrome !!
Time Complexity: O(n)