leetcode Valid Palindrome

Valid Palindrome

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Question Solution 

Total Accepted: 68173 Total Submissions: 309046 Difficulty: Easy

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.

该题主要是理解问题是什么。所谓的回串就是这个串的第一个和最后一个相等，第二个和倒数第二个相等.......当然，不区分大小写，如果是非数字非字母则跳过。