String Palindrome Check (two pointers, normalize)

Palindrome checks on strings compare mirrored positions—two pointers from both ends moving inward—or compare s to s[::-1] after normalizing spaces and punctuation. Odd-length ignores the middle character.

Why this shows up in the real world

DNA reverse complement checks symmetry-like properties on sequences. Chat moderation flags palindrome spam patterns.

Core idea (explained for students)

Two-pointer: L, R = 0, len(s)-1; loop while L < R, skip non-alnum if ignoring punctuation, compare s[L].lower()==s[R].lower(), then move pointers.

Try this in Python

def is_palindrome_simple(s: str) -> bool:
    t = "".join(c.lower() for c in s if c.isalnum())
    return t == t[::-1]


print(is_palindrome_simple("A man, a plan, a canal: Panama"))

Common mistakes

• Unicode casefold vs lower for international text.
• Off-by-one when L == R at middle—still a palindrome.

Key takeaways

• Choose normalization rules (ignore spaces? digits only?) before coding.
• s == s[::-1] is acceptable when normalized string is small.