Reverse a string with special characters preserved.

Why Preserve Special Chars: Log parsing (reverse for suffix search), obfuscation (mask emails but keep structure), and data cleaning where delimiters must stay.

Two-Pointer: Swap only when both i and j point to letters. Advance when hitting non-letter. O(n), single pass. Convert to list for in-place; join for result.

Edge Cases: Empty string, all special chars, Unicode (use str.isalpha()—handles most; unicodedata for full Unicode).

def reverse_preserve_special(s):
lst, i, j = list(s), 0, len(s)-1
while i < j:
if not lst[i].isalpha(): i += 1
elif not lst[j].isalpha(): j -= 1
else:
lst[i], lst[j] = lst[j], lst[i]
i += 1; j -= 1
return ''.join(lst)