问题描述：

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.

代码：

import java.util.Vector;


public class Valid_Palindrome {
	public boolean isPalindrome(String s) {
		s = s.toLowerCase();
		Vector<Character> vector = new Vector<Character>();
        for(int i = 0; i< s.length(); i++){
        	Character ch = s.charAt(i);
        	if((ch >='a' && ch <='z' )||(ch >='0' && ch <='9'))
        		vector.add(ch);
        }
        
        if(vector.size() == 0)
        	return true;
        
        int size = vector.size();
        for(int i=0; i< size/2; i++){
        	if(vector.get(i) != vector.get(size-1-i))
        		return false;
        }
        return true;
    }
}

[leetcode]Valid Palindrome