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.

思路：

1. 必须把字符串统一为小写，而且要忽略除数字和字符外的所有符号。

2. 利用 String.toCharArray() 转换字符串为Character数组。

3. 双指针check头尾，直到双指针相遇前停止，不相等则返回false。

4. 结束check则返回true。