Your task is to produce a set of all letters in a string. Letters are represented as strings of length 1. For example, if you are given a string

Mary had a little lamb

you should produce the set

{ " ", "M", "a", "b", "d", "e", "h", "i", "l", "m", "r", "t", "y" }

Complete the following file:

import java.util.Set; import java.util.TreeSet; public class LetterSet { public Set<String> makeLetterSet(String str) { // TODO: Complete method } }