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.

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        start, end = 0, len(s) - 1
        while start < end:
            if not s[start].isalnum():
                start += 1
                continue
            if not s[end].isalnum():
                end -= 1
                continue
            if s[start].lower() != s[end].lower():
                return False
            start += 1
            end -= 1
        return True