Write a method that reverses only the alphabetic characters in a string while keeping special characters and numbers in their original positions.
Examples
Input: str = 'a,b$c'
Output: 'c,b$a'
Explanation: Only 'a', 'b', 'c' are reversed to 'c', 'b', 'a'. The comma and dollar sign stay in positions 1 and 3.
Input: str = 'Ab,c,de!$'
Output: 'ed,c,bA!$'
Explanation: Letters 'A', 'b', 'c', 'd', 'e' reverse to 'e', 'd', 'c', 'b', 'A'. Special characters stay in place.
Input: str = '123'
Output: '123'
Explanation: No alphabetic characters to reverse, string remains unchanged.
Input: str = 'hello'
Output: 'olleh'
Explanation: All characters are alphabetic, entire string is reversed.
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