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。

 1     public boolean isPalindrome(String s) {
 2         if (s.isEmpty()) return true;
 3         int begin = 0,end = s.length()-1;
 4         while (begin<=end)
 5         {
 6             Character bchar = s.charAt(begin);
 7             Character echar = s.charAt(end);
 8             if (!Character.isLetterOrDigit(bchar)) begin++;
 9             else if (!Character.isLetterOrDigit(echar)) end--;
10             else if (Character.toLowerCase(bchar) != Character.toLowerCase(echar)) return false;
11             else {
12                 begin++;end--;
13             }
14         }
15         return true;        
16     }