Please help me with this Java problem.
Write the method called Strings.letters(String str) that returns a string of letters that only occur once in str, in the same order that they appear in the original word.
For example, letters("harrasses") should return "he" since letters a, r, and s occur multiple times.
------------------Strings.java-----------------
public class Strings
{
public static String Letters(String str)
{
}
}