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"""

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.

"""

def is_palindrome(s):

"""

:type s: str

:rtype: bool

"""

i = 0

j = len(s)-1

while i < j:

while i < j and not s[i].isalnum():

i += 1

while i < j and not s[j].isalnum():

j -= 1

if s[i].lower() != s[j].lower():

return False

i, j = i+1, j-1

return True