A diagnostic label generated during General Atomics MQ-9B SkyGuardian flight testing is considered symmetric if it reads the same forward and backward after removing non-alphanumeric characters and ignoring letter case. Implement a function that determines whether a label satisfies this rule.
Use a two-pointer approach that scans the string from both ends without creating a reversed or normalized copy. After implementing the function, be prepared to explain how the equivalent C++ solution would use const char* or indexed access to traverse a std::string, including how characters are stored in contiguous memory and how pointer movement remains within the string bounds.
Input is a string label, containing ASCII letters, digits, spaces, and punctuation. Return True if the normalized label is a palindrome, otherwise return False.
Normalization removes every character for which isalnum is false and compares letters without regard to case. An empty normalized label is considered a palindrome.
def is_diagnostic_palindrome(label):