Valid Palindrome

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.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

public class Solution {
    public boolean isPalindrome(String s) {
        if (s==null||s.length()==0) {
            return true;
        }
        char a = 'a';
        char z = 'z';
        int begin = 0,end = s.length()-1;
        //这里只能用一次不能再循环里调用 不然会TLE
        char[] chars = s.toCharArray();
        while (begin<=end) {
            char charBegin = chars[begin];
            //处理大小写转换问题，统一转小写
            if (charBegin>='A'&&charBegin<='Z') {
                charBegin = (char) (charBegin + 32);
            }
            char charEnd = chars[end];
            if (charEnd>='A'&&charEnd<='Z') {
                charEnd = (char) (charEnd + 32);
            }
            //不是可用的字符跳过 注意边界没有 =
            if (charBegin < a || charBegin > z) {
                if (charBegin<'0' || charBegin>'9') {
                    begin++;
                    continue;
                }
            }
            if (charEnd < a || charBegin > z) {
                if (charEnd<'0' || charEnd>'9') {
                    end--;
                    continue;
                }
            }
            if (charBegin != charEnd) {
                return false;
            }
            begin++;end--;
        }
        return true;
    }
}