public void Question03ShouldRemoveDuplicateCharacters()
{
string test1 = "abcd";
Question03.RemoveDuplicateCharacters(ref test1);
string test2 = "abcdd";
Question03.RemoveDuplicateCharacters(ref test2);
string test3 = "abc d ";
Question03.RemoveDuplicateCharacters(ref test3);
string test4 = null;
Question03.RemoveDuplicateCharacters(ref test4);
string test5 = "aaaa";
Question03.RemoveDuplicateCharacters(ref test5);
string test6 = "ababab";
Question03.RemoveDuplicateCharacters(ref test6);
Assert.Equal("abcd", test1);
Assert.Equal("abcd", test2);
Assert.Equal("abc d", test3);
Assert.Null(test4);
Assert.Equal("a", test5);
Assert.Equal("ab", test6);
}
