a function that takes a string as an input checks the letters of the input and returns a list that contains the characters in the string only once and in alphabetical order.

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```python # define a function named unique_sorted_characters def unique_sorted_characters(input_string): # use set to remove duplicate characters and sorted to sort them return sorted(set(input_string)) ``` ```python # main function that tests unique_sorted_characters def main(): # test the unique_sorted_characters function # use case 1: input string with duplicates assert unique_sorted_characters("hello") == ['e', 'h', 'l', 'o'] # use case 2: input string with no duplicates assert unique_sorted_characters("world") == ['d', 'l', 'o', 'r', 'w'] # call the main function to run the tests main() ``` ∻Explanation∻ ⚹The function `unique_sorted_characters` takes a string as input, converts it to a set to remove duplicates, then converts the set to a sorted list, and returns it. The main function `main` tests this function with two use cases to ensure it works as expected.⚹ ∻Step-by-step Instruction∻ ‖ ▪ ⋮ Define the main function `unique_sorted_characters` that processes the input string. ‖ ‖ ▪ ⋮ Use a set to remove duplicate characters from the string. ‖ ‖ ▪ ⋮ Use the `sorted` function to sort the unique characters in alphabetical order. ‖ ‖ ▪ ⋮ Define the `main` function to test `unique_sorted_characters` with specific use cases. ‖ ‖ ▪ ⋮ Use assertions to test the correctness of the function with different input strings. ‖ ∻Time Complexity∻ ⚹The time complexity of the function is O(n log n), where n is the length of the string, due to the sorting operation.⚹ ∻Space Complexity∻ ⚹The space complexity is O(n), where n is the length of the string, as it may contain all unique characters.⚹

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