题目：
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" isnot
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.
给定一个字符串，确定它是否是一个回文，只考虑忽略字母数字字符的情况。
思路：
之前有遇到过判断回文的情况，只要判断前后字符是否相同就好，但这里，加了符号汉字，但原理还是一样的，定义两个指针，从开头和结尾开始遍历，遇到汉字或者符号就跳过，遇到字母数字就进行比较。查看解决方案。用IsDigit来判断数字，IsLetter判断字母。