Is a message code a clean palindrome

Question

A confirmation code should read identically forwards and backwards once you ignore punctuation, spaces, and letter case. Decide whether a given string is such a palindrome.

s = "Ab, ba!"
-> True          # cleaned to "abba"

Return True if the alphanumeric characters form a palindrome (case-insensitive), else False. Aim for $O(n)$ time and $O(1)$ extra space.

Accepted Answer

Walk two pointers inward from both ends. Skip any character that is not alphanumeric on either side, then compare the two current characters case-insensitively. Any mismatch means it is not a palindrome; if the pointers cross without a mismatch, it is. Tracing "Ab, ba!": the pointers skip the comma, space, and exclamation, comparing a==a and b==b. No mismatch, so the answer is True. Time O(n), space O(1), no cleaned copy is built. Edge cases: an empty string or one with no alphanumerics is vacuously a palindrome (returns True), and case folding must happen at comparison time. Takeaway. Two po…

Is a message code a clean palindrome

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