Leetcode(Medium) - Top K Frequent Elements

Question

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

<Example 1>

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

<Example 2>

Input: nums = [1], k = 1
Output: [1]

<Constraints>

• 1 <= nums.length <= 10^5
• -10^4 <= nums[i] <= 10^4
• k is in the range [1, the number of unique elements in the array].
• It is guaranteed that the answer is unique.

Solution

# Time Complexity: O(klogN)
class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        my_dic = {}
        res = []

        for num in nums:
            my_dic[num] = 1 + my_dic.get(num, 0)

        heap = [(-value, key) for key, value in my_dic.items()]
        heapq.heapify(heap)

        for _ in range(k):
            value, key = heapq.heappop(heap)
            res.append(key)

        return res

# Time Complexity: O(n)
class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        my_dic = {}
        frequent = [[] for i in range(len(nums) + 1)]

        for num in nums:
            my_dic[nums] = 1 + my_dic.get(num, 0)

        for value, count in my_dic.items():
            frequent[count].append(value)

        result = []
        for idx in range(len(freq)-1, 0, -1):
            for value in freq[idx]:
                result.append(value)
                if len(result) == k:
                    return result

Time/Space Complexity

• Time Complexity: O(n)
• Space Complexity: O(n)