[leetcode][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"

 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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import string
class Solution(object):
def isPalindrome(self, s):
"""
:type s: str
:rtype: bool
"""
valid = set(string.ascii_lowercase+ string.digits)

left = 0
right = len(s) -1
while left < right:
s_l = s[left].lower()
s_r = s[right].lower()

if (s_l in valid) and (s_r in valid):

if s_l == s_r:
left += 1
right -= 1
else:
return False

elif s_l in valid:
right -= 1
elif s_r in valid:
left += 1
else:
left += 1
right -= 1
return True