125. 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.

题目含义：给定一个字符串，判断是不是回文，只考虑字母数字字符，并不顾大小写。另外，空串被认为是有效的回文。

    public boolean isPalindrome(String s) {
        if (s.isEmpty()) return true;
        int begin = 0,end = s.length()-1;
        while (begin<=end)
        {
            Character bchar = s.charAt(begin);
            Character echar = s.charAt(end);
            if (!Character.isLetterOrDigit(bchar)) begin++;
            else if (!Character.isLetterOrDigit(echar)) end--;
            else if (Character.toLowerCase(bchar) != Character.toLowerCase(echar)) return false;
            else {
                begin++;end--;
            }
        }
        return true;        
    }