Added some Amazon interview questions

1. Given a singly linked list of integers, determine if there is a loop.

2. Given a list of TestResult objects, which each contain a studentId,
test date and test score, and assuming each student has taken at least 5
tests, determine each student's final score. The final score is
determined by the average of the student's 5 highest scores.

3. Given two unsorted arrays, determine the intersection (elements which
appear in both arrays) of the arrays.