LeetCode 125 Valid Palindrome - Implementation

Two pointers skip non-alphanumeric characters and compare case-insensitively.

function isPalindrome(s):
    left = 0
    right = len(s) - 1
    while left < right:
        while left < right and not isAlphanumeric(s[left]):
            left += 1
        while left < right and not isAlphanumeric(s[right]):
            right -= 1
        if toLower(s[left]) != toLower(s[right]):
            return false
        left += 1
        right -= 1
    return true

$O(n)$ time, $O(1)$ space.