125.isPalindrome.py

# Example 1:

# Input: s = "A man, a plan, a canal: Panama"
# Output: true
# Explanation: "amanaplanacanalpanama" is a palindrome.
# Example 2:
#
# Input: s = "race a car"
# Output: false
# Explanation: "raceacar" is not a palindrome.
# Example 3:
#
# Input: s = " "
# Output: true
# Explanation: s is an empty string "" after removing non-alphanumeric characters.
# Since an empty string reads the same forward and backward, it is a palindrome.


class Solution:

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

    def alphaNum(self, c):
        # check if c is alphanumeric
        return (
            ord("A") <= ord(c) <= ord("Z")
            or ord("a") <= ord(c) <= ord("z")
            or ord("0") <= ord(c) <= ord("9")
        )


if __name__ == "__main__":
    s = Solution()
    print(s.isPalindrome("A man, a plan, a canal: Panama"))