Given a string, find the longest substring without repeating characters.
For example, for string “abccdefgh”, the longest substring is “cdefgh”.

​import java.util.HashSet;

public class Substring {

 public static void main(String[] args) {
//  System.out.println(longest("abccdefgh"));
//  System.out.println(longest(""));
//  System.out.println(longest("cd"));
//  System.out.println(longest("cc"));
  System.out.println(longest("abcdefghabcdefgh"));
 }

 public static String longest(String str) {

  if(str != null && str.length() > 0) {

   HashSet<Character> hs = new HashSet<>();
   HashSet<Character> hsTemp = new HashSet<>();
   for(char ch : str.toCharArray()) {
    if(!hsTemp.add(ch) && hsTemp.size() > hs.size()) {
     hs.clear();
     hs.addAll(hsTemp);
     hsTemp.clear();
     hsTemp.add(ch);
    }    
   }
   if(hs.isEmpty() || hsTemp.size() > hs.size()) {
    StringBuilder sb = new StringBuilder();
    for(char ch : hsTemp)
     sb.append(ch);
    return sb.toString();
   }
   else {
    StringBuilder sb = new StringBuilder();
    for(char ch : hsTemp)
     sb.append(ch);
    return sb.toString();
   }
  } else {
   return null;
  }
 }

}