"""
Reverse Only Letters in a List Implementation in Python.
"""
from typing import List
def reverse_only_letters(chars: List[str]) -> None:
"""
Reverses only the alphabetic characters in a list of characters in-place,
leaving special characters, spaces, and digits in their original positions.
Args:
chars (List[str]): The input list of characters to modify.
"""
if not chars:
return
left = 0
right = len(chars) - 1
# Use a two-pointer approach moving inward from both ends
while left < right:
# Advance left pointer until it points to an alphabetic character
while left < right and not chars[left].isalpha():
left += 1
# Move right pointer backward until it points to an alphabetic character
while left < right and not chars[right].isalpha():
right -= 1
# Swap the valid letters found at both pointers using Python's tuple assignment
if left < right:
chars[left], chars[right] = chars[right], chars[left]
left += 1
right -= 1
def main() -> None:
# Input list
lst = ['a', '#', 'b', '$', '%', 'c', '&', '*', '(', 'd', 'e', 'f', '!']
print(f"Original: {lst}")
# Apply the reversal function in-place
reverse_only_letters(lst)
print(f"Reversed: {lst}")
if __name__ == "__main__":
main()
'''
run:
Original: ['a', '#', 'b', '$', '%', 'c', '&', '*', '(', 'd', 'e', 'f', '!']
Reversed: ['f', '#', 'e', '$', '%', 'd', '&', '*', '(', 'c', 'b', 'a', '!']
'''
