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.
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Solve this problem
去掉S中的无关字符,将大写字母转换为小写,之后就是简单的回文串判断。
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