John has a string, consisting of n English alphabetic letters. He needs to compact the string such that any adjacent pairs of letters will be represented as a single character. 

Input:aabbccabddf, Output: abcabdf

Input:aabbaa, Output: aba

First step should be to demonstrate like you understood the task. You can write your own example and ask questions - did you understand ?

You should start to ask right question:

- is the string case sensitive?

- should we validate the string?

- what we should return after validation? IllegalArgumentException? 

Next step is explain your approach. If you will use pseudo code or will explain the technology you will be use - it would be perfect start. 

 What is the goal of this interview? to show you are familiar with the best coding practice :

    - coding style,
    - one method - one responsibility,
    - security understanding (private/public),
    - knowledge of the most efficient way to resolve / about libraries,
    - thinking about testing and validation,
    - hands-on coding development. 

If you could you features of java 8+, junit library to test - it would be good sign for you.

In the example we provided you could show your knowledge of regular expression.

private static String compactString(String s){
   while(true){
     int length = s.length();
     s=s.replaceAll("(.)(\\1+)","$1");
     if (s.length()==length) {
         break;
     }
     return s;
   }
}