Valid palindrome -125

class Solution {
    public boolean isPalindrome(String s) {
        s=s.toLowerCase();
        StringBuilder b = new StringBuilder();
        for(int i=0;i<s.length();i++){
            if(Character.isDigit(s.charAt(i))||Character.isLetter(s.charAt(i))){
                b.append(s.charAt(i));
            }
        }
        
        int begin = 0;
        int end = b.length()-1;
        while(begin<end){
            if(b.charAt(begin)!=b.charAt(end))
                return false;
            begin++;
            end--;
        }
        return true;
    }
}