Day12. Valid Palindrome
LeetCode125. Valid Palindrome
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.
判断字符串是否是回文字符串，字符串中除了数字字符和字母之外，其他字符不计，大写字母和小写字母视为同种字符。
头尾两边对比判断就行了。空字符串视为合法回文串。