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347. Top K Frequent Elements

Description

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 <= 105
  • -104 <= nums[i] <= 104
  • k is in the range [1, the number of unique elements in the array].
  • It is guaranteed that the answer is unique.

 

Follow up: Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

Solutions

Solution 1: Hash Table + Priority Queue (Min Heap)

We can use a hash table $\textit{cnt}$ to count the occurrence of each element, and then use a min heap (priority queue) to store the top $k$ frequent elements.

First, we traverse the array once to count the occurrence of each element. Then, we iterate through the hash table, storing each element and its count into the min heap. If the size of the min heap exceeds $k$, we pop the top element of the heap to ensure the heap size is always $k$.

Finally, we pop the elements from the min heap one by one and place them into the result array.

The time complexity is $O(n \log k)$, and the space complexity is $O(k)$. Here, $n$ is the length of the array.

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class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        cnt = Counter(nums)
        return [x for x, _ in cnt.most_common(k)]

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class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        Map<Integer, Integer> cnt = new HashMap<>();
        for (int x : nums) {
            cnt.merge(x, 1, Integer::sum);
        }
        PriorityQueue<Map.Entry<Integer, Integer>> pq
            = new PriorityQueue<>(Comparator.comparingInt(Map.Entry::getValue));
        for (var e : cnt.entrySet()) {
            pq.offer(e);
            if (pq.size() > k) {
                pq.poll();
            }
        }
        return pq.stream().mapToInt(Map.Entry::getKey).toArray();
    }
}

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class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        unordered_map<int, int> cnt;
        using pii = pair<int, int>;
        for (int x : nums) {
            ++cnt[x];
        }
        priority_queue<pii, vector<pii>, greater<pii>> pq;
        for (auto& [x, c] : cnt) {
            pq.push({c, x});
            if (pq.size() > k) {
                pq.pop();
            }
        }
        vector<int> ans;
        while (!pq.empty()) {
            ans.push_back(pq.top().second);
            pq.pop();
        }
        return ans;
    }
};

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func topKFrequent(nums []int, k int) []int {
    cnt := map[int]int{}
    for _, x := range nums {
        cnt[x]++
    }
    pq := hp{}
    for x, c := range cnt {
        heap.Push(&pq, pair{x, c})
        if pq.Len() > k {
            heap.Pop(&pq)
        }
    }
    ans := make([]int, k)
    for i := 0; i < k; i++ {
        ans[i] = heap.Pop(&pq).(pair).v
    }
    return ans
}

type pair struct{ v, cnt int }
type hp []pair

func (h hp) Len() int           { return len(h) }
func (h hp) Less(i, j int