Top K Frequent Elements

The problem. Given an integer array nums and an integer k, return the k most frequent values. The answer may be in any order, and it is guaranteed to be unique.

Example

nums = [1, 1, 1, 2, 2, 3],         k = 2  ->  [1, 2]
nums = [1],                        k = 1  ->  [1]
nums = [4, 1, -1, 2, -1, 2, 3],    k = 2  ->  [-1, 2]

Every approach starts the same way: one pass with a hash map to count each value. The question is how to pick the k largest counts.