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.

Similar Questions: Palindrome Linked List 

Next challenges: Max Consecutive Ones II Detect Capital Design Log Storage System

思路：要比较的是字符串中的数字和字母，注意字母的大小写是不区分的。头尾2个指针，遇到非数字或者非字母的位置就直接跳过。Java中Character.isLetterOrDigit()来判断当前字符是否为数字或者字母，Character.toLowerCase()将字母统一转换为小写，数字不变。

代码：

 1 public class Solution {
 2     public boolean isPalindrome(String s) {
 3         int len = s.length();
 4         for(int start = 0, end = len - 1; end > start; ) {
 5             if(!Character.isLetterOrDigit(s.charAt(start))) {
 6                 start++;
 7                 continue;
 8             }
 9             if(!Character.isLetterOrDigit(s.charAt(end))) {
10                 end--;
11                 continue;
12             }
13             if(Character.toLowerCase(s.charAt(start)) != Character.toLowerCase(s.charAt(end))) return false;
14             start++;
15             end--;
16         }
17         return true;
18     }
19 }

Leetcode 125. Valid Palindrome