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：给定一个字符串，检测其是否是回文串，只考虑字母数字字符。

思路：过滤源字符串中的非字母数字，组成一个新串，对新串进行判断。

	public static boolean isPalindrome(String s) {
		if(s.isEmpty())
			return true;
		//过滤字母数字之外的字符
		StringBuffer buf = new StringBuffer();
		for(int i=0;i<s.length();i++){
			if(Character.isLetterOrDigit(s.charAt(i)))
				buf.append(s.charAt(i));
		}
		String tmp = buf.toString().toLowerCase();
		for(int i=0;i<tmp.length();i++){
			if(tmp.charAt(i) != tmp.charAt(tmp.length() - i - 1))
				return false;
		}
		return true;
	}