Counting Duplicates

 Problem: Given an array of n integers, count the number of duplicate integers in the array.

Example:

Input [1,2,3,1]

Output True

Input [1,2,3,4]

Output False

Input [1,2,3,3]

Output True

Solution: Using a HashSet, if the integer exists in the HashSet, return True else add that integer in to the HashSet.