class Solution:
def topKFrequent(self, nums, k):
"""
:type nums: List[int]
:type k: int
:rtype: List[int]
"""
"""
Counter is a subclass of dictionary
Time Complexity:
construct: O(n)
the operator of most_common uses heapq to obtain top k value O(1)
zip: O(n)
"""
counter = collections.Counter(nums)
res = list(zip(*counter.most_common(k)))[0]
return list(res)
