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
class Solution:
     def isPalindrome(self, s):
         """
         :type s: str
         :rtype: bool
         """
         s = s.lower()
         s = [c for c in s if c.isalnum()]
         return s == s[::-1]

class Solution:
     def isPalindrome(self, s):
         """
         :type s: str
         :rtype: bool
         """
         s = s.lower()
         s = [c for c in s if c.isalnum()]
         return s == s[::-1]
