基本思路就是从两边想中间挤压。记得前不久有一道题使用了相似的处理手段，容我有时间找找看！
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
Note: For the purpose of this problem, we define empty string as valid palindrome.
Example 1:
Input: "A man, a plan, a canal: Panama" Output: true
Example 2:
Input: "race a car" Output: false
link:{sourcedir}/_0125_ValidPalindrome.java[role=include]