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.

其中可以在以后刷题中用到的：

Character.isLetterOrDigit()
Character.toLowerCase()

public class Solution {
    public boolean isPalindrome(String s) {
        if(s==null)
        	return true;
        int len=s.length();
        int p=0;
        int q=len-1;
        while(p<q)
        {
        	while(Character.isLetterOrDigit(s.charAt(p))==false&&p<q)
        	{
        		p++;
        	}
        	while(Character.isLetterOrDigit(s.charAt(q))==false&&p<q)
        	{
        		q--;
        	}
        	if(Character.toLowerCase(s.charAt(p))!=Character.toLowerCase(s.charAt(q)))
        	{
        		return false;
        	}
        	else
        	{
        		p++;
        		q--;
        	}
        }
        return true;	
    }
}