Pergunta de entrevista da empresa NIKE

The first phone screen asked about overall experiences, working process, software qualify, and also problem solving (find the most duplicate word in a string, using HashMap) and if the string is too long. Let's say million of character length or even bigger than that. How do you solve? How do you test this method? The 2nd interview is about giving a problem business problem, design a rest api? tell them where you will start? what is the corner case? how do you test? How much test is enough?