Given a non-empty array of integers, return the k most frequent elements.
Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2]
Input: nums = , k = 1 Output: 
- You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
- Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size.
We use a hashmap to count every number. Then we transform this hashmap to a list, and sort it, output the first k elements.
To see more hashtable related problems, see
Problems and Solutions of Hashmap.