Combine Strings and Remove Duplicates in Python

The challenge

Take 2 strings s1 and s2 including only letters from ato z. Return a new sorted string, the longest possible, containing distinct letters – each taken only once – coming from s1 or s2.

Examples:

a = "xyaabbbccccdefww"
b = "xxxxyyyyabklmopq"
longest(a, b) -> "abcdefklmopqwxy"

a = "abcdefghijklmnopqrstuvwxyz"
longest(a, a) -> "abcdefghijklmnopqrstuvwxyz"

The solution in Python code

Option 1 (Using a Dictionary):

def longest(a1, a2):
    a3 = list(dict.fromkeys(sorted(a1+a2)))
    return "".join(a3)

Option 2 (Using a Set):

def longest(a1, a2):
    return "".join(sorted(set(a1 + a2)))

Option 3 (Using a Union):

def longest(s1, s2):
    return ''.join(sorted(set(s1).union(s2)))

Test cases to validate our solution

import test as test
    
@test.describe("longest")
def tests():
    @test.it("basic tests")
    def basics():
        test.assert_equals(longest("aretheyhere", "yestheyarehere"), "aehrsty")
        test.assert_equals(longest("loopingisfunbutdangerous", "lessdangerousthancoding"), "abcdefghilnoprstu")
        test.assert_equals(longest("inmanylanguages", "theresapairoffunctions"), "acefghilmnoprstuy")