Leetcode (Python): 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.

Solution:

class Solution:
    # @param s, a string
    # @return a boolean
    def isPalindrome(self, s):
        start = 0;
        end = len(s)-1
        while start<end:
            while start<end and not s[start].isalnum():
                start +=1
            while start<end and not s[end].isalnum():
                end -=1
            if start<end and s[start].lower() != s[end].lower():
                return False
            start +=1
            end -= 1
        return True