Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases. For example, “A man, a plan, a canal: Panama” is a palindrome. “race a car” is not a palindrome. ~~~ public class Solution { public boolean isPalindrome(String s) { if (s == null) { return false; } if (s.length() == 0 || s.length() == 1) { return true; } StringBuffer sb = new StringBuffer(); for (int i = 0; i < s.length();i++){ Character ch = s.charAt(i); //if(Character.isLetter(ch)){ if(Character.isLetter(ch) || Character.isDigit(ch)){ sb.append(Character.toUpperCase(ch)); } else { continue; } } int start = 0 ; int end = sb.length() - 1; while ( start < end) { if (sb.charAt(start) == sb.charAt(end)) { start ++; end --; } else { return false; } } return true; } } ~~~