:type s: str
    :rtype: bool

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