class Solution {
public boolean isPalindrome(String s) {
if (s == null) return true;
s = s.toLowerCase();
int l = s.length();
StringBuilder str = new StringBuilder(l);
for (char c : s.toCharArray()) {
if ((c >= '0' && c <= '9') || (c >= 'a' && c <= 'z')) {
str.append(c);
}
}
return str.toString().equals(str.reverse().toString());
}
}