"""
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
        """
        if len(s)<=1:
            return True
        chars=[]
        for i in range(len(s)):
            if s[i] >= 'a' and s[i] <= 'z' or s[i] >= '0' and s[i] <= '9' or s[i] >= 'A' and s[i] <= 'Z':
                chars.append(s[i].lower())
        left,right=0,len(chars)-1
        while left<=right:
            if chars[left]!=chars[right]:
                return False
            else:
                left,right=left+1,right-1
        return True
