Valid Palindrome

class Solution:
    def isPalindrome(self, s: str) -> bool:
        def check(ch):
            if ch.isdigit():
                return ch
            elif ch.isalpha():
                return ch.lower()
            return ''

        N = len(s)
        l, r = 0, N - 1
        flag = True
        while l <= r:
            while l < N and not check(s[l]):
                l += 1
            while r >= 0 and not check(s[r]):
                r -= 1

            if l <= r:
                flag &= check(s[l]) == check(s[r])

            l += 1
            r -= 1
        return flag