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125 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"isnota 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.
The Idea: Confirm a regular palindrome, but ignore non alpha numeric characters.
Complexity: O(n) time and O(1) space
class Solution:
def isPalindrome(self, s):
:type s: str
:rtype: bool
def isalpha_or_digit(char):
return char.isalpha() or char.isdigit()
left = 0
right = len(s) - 1
while left < right:
# get the first valid left character
while left < right and not s[left].isalnum():
left += 1
# get the first valid right character:
while left < right and not s[right].isalnum():
right -= 1
if s[left].lower() != s[right].lower():
return False
left += 1
right -= 1
return True