Java [Leetcode 125]Valid Palindrome

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;
    }
}