How can you remove duplicate characters in a string?

Question

I have to implements a function that takes a string as an input and finds the non-duplicate character from this string.

So an an example is if I pass string str = \"

Answer 1

Not sure how optimal it is:

public static string RemoveDuplicates(string input)
{
    var output = string.Join("", input.ToHashSet());
    return output;
}