Refactor and expand is_palindrome test cases

Added clarification comments on `isalnum()` and its implementation. Extended test cases to cover additional edge cases, improving robustness.

Author: nicholasadamou

Two Pointers/valid_palindrome.py

@@ -1,5 +1,20 @@
 # https://neetcode.io/problems/is-palindrome
 
+# `isalnum()` method returns True if all the characters are alphanumeric, meaning alphabet letter (a-z) and numbers (0-9).
+# `isalpha()` method returns True if all the characters are alphabet letters (a-z).
+# `isdigit()` method returns True if all the characters are digits (0-9).
+# `islower()` method returns True if all the characters are in lower case.
+
+# Therefore, for this problem, we can use `isalnum()` to check if the character is an alphabet letter or a number.
+# However, if we are asked to implement `isalnum()` method, we can use the following code:
+
+# def is_alnum(character: str) -> bool:
+#     return (
+#          ord('A') <= ord(character) <= ord('Z') or
+#          ord('a') <= ord(character) <= ord('z') or
+#          ord('0') <= ord(character) <= ord('9')
+#    )
+
 import unittest
 
 
@@ -34,3 +49,4 @@ class Test(unittest.TestCase):
     def test_is_palindrome(self):
         self.assertTrue(is_palindrome("Was it a car or a cat I saw?"))
         self.assertFalse(is_palindrome("tab a cat"))
+        self.assertFalse(is_palindrome("0P"))