class validPalin { public static boolean isPalindrome(String s) { // convert to lower case s = s.toLowerCase(); // build a string with solely char or dig StringBuilder sb = new StringBuilder(); for (char elem : s.toCharArray()) { if (Character.isLetterOrDigit(elem)) { sb.append(elem); } } String alNumString = sb.toString(); // redundant int r = alNumString.length() - 1; int l = 0; while (r > l) { if (alNumString.charAt(l) != alNumString.charAt(r)) { return false; } r--; l++; } return true; } public static void main(String[] args) { String s1 = "A man, a plan, a canal: Panama"; System.out.println("Output1: " + isPalindrome(s1)); String s2 = "race a car"; System.out.println("Output2: " + isPalindrome(s2)); String s3 = " "; System.out.println("Output3: " + isPalindrome(s3)); } } /* https://leetcode.com/problems/valid-palindrome/description/ start at both ends; each element must equal each other after downcase and if alphanumeric * A phrase is a palindrome if, after converting all uppercase letters * into lowercase letters and removing all non-alphanumeric characters, * it reads the same forward and backward. Alphanumeric characters * include letters and numbers. Given a string s, return true if it is a palindrome, or false otherwise. Example 1: Input: s = "A man, a plan, a canal: Panama" Output: true Explanation: "amanaplanacanalpanama" is a palindrome. Example 2: Input: s = "race a car" Output: false Explanation: "raceacar" is not a palindrome. Example 3: Input: s = " " Output: true Explanation: s is an empty string "" after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome. */