def reverse_alternating_groups(s):
    result = []  # List to store the final result
    group = ""   # Temporary string to hold each group (either digits or letters)
    reverse = False  # Flag to indicate whether to reverse the group

    for i in range(len(s)):
        if s[i].isalpha():
            # Add character to the current group of letters
            group += s[i]
        else:
            # If we encounter a non-letter (digit), process the current group
            if group:
                # Reverse the group if needed and append it to the result
                if reverse:
                    result.append(group[::-1])  # Reverse the letters group
                else:
                    result.append(group)  # Leave the letters group as it is
                group = ""  # Reset the group for the next set of characters

            # Append the digit to the result (digits are unchanged)
            result.append(s[i])

            # Toggle the reverse flag for the next group of letters
            reverse = not reverse

    # Process any remaining group (in case the string ends with letters)
    if group:
        if reverse:
            result.append(group[::-1])  # Reverse the last letters group if needed
        else:
            result.append(group)

    # Return the result as a joined string
    return ''.join(result)

# Example usage:
input_str_1 = "abc123def456ghi789jkl"
output_str_1 = reverse_alternating_groups(input_str_1)
print(output_str_1)  # Output: "cba123def456ihg789jkl"

input_str_2 = "a1b2c3d"
output_str_2 = reverse_alternating_groups(input_str_2)
print(output_str_2)  # Output: "a1b2c3d"