leetcode_125 Valid Palindrome(Java)

熟悉以下函数的用法：
(1) s.toLowerCase()
(2) StringBuilder str = new StringBuilder(len)
(3) s.toCharArray()
(4) str.append(…)
(5) str.toString().equals(str.reverse().toString())

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