A phrase is a palindrome if, after converting all uppercase letters to lowercase and removing all non-alphanumeric characters, it reads the same forward and backward. Given a string s, return true if it is a palindrome.

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Pythontime O(n) · space O(1)

def isPalindrome(s: str) -> bool:
    l, r = 0, len(s) - 1
    while l < r:
        while l < r and not s[l].isalnum(): l += 1
        while l < r and not s[r].isalnum(): r -= 1
        if s[l].lower() != s[r].lower(): return False
        l, r = l + 1, r - 1
    return True

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