题目描述：
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.
解题思路：
设置两个指针，一个在字符串头部，一个在字符串尾部，分别向中间移动，遇到非字母或数字则继续向中间移动，如两个都为字母或者数字，那么则比较两者是否相同。
代码如下：
public class Solution {
public boolean isPalindrome(String s) {
int i = 0, j = s.length() - 1;
char head, tail;
if(j < 0)
return true;
while(i < j){
head = s.charAt(i);
tail = s.charAt(j);
if(!Character.isLetterOrDigit(head)){
i++;
}
if(!Character.isLetterOrDigit(tail)){
j--;
}
if(Character.isLetterOrDigit(head) && Character.isLetterOrDigit(tail)){
if(Character.toLowerCase(head) != Character.toLowerCase(tail)){
return false;
}
i++;
j--;
}
}
return true;
}
}