Longest substring with two Unique Characters

Write a method to find the longest substring that is composed of any two unique repeating characters.

Examples: 

longestSubstr("acacacacggtttttacacacacggcaca") ==> acacacac

longestSubstr("ababajfhgsjkdlsvcgabaaaaababahhhhbab") ==> abaaaaababa

Need a hand? Try out these hints, one at a time.

public static String longestSubstr(String s)  {
  



}

Java

Python