[LeetCode] 347. Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note: 

• 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.

Subscribe to see which companies asked this question.

用內建的工具 collections 和 most_common 

line 6: 因為counter 是一個字典, 在這裡 i 是pair (key, value). 所以 i[0] = key

另一種寫法 用operator