Top K Frequent Elements

Heap

Top K Frequent Elements

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

Example 1:

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

Example 2:

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

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.
It’s guaranteed that the answer is unique, in other words the set of the top k frequent elements is unique.
You can return the answer in any order.

C++

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {

        // Map to store element and their frequencies
        unordered_map<int, int> counts;

        for(auto num: nums)
            counts[num]++;

        // Heap that sorts first based on frequency and 
        // then based on element
        set<pair<int, int>> countHeap;

        // Populate the heap
        for(auto item: counts)
            countHeap.insert({ item.second, item.first });

        vector<int> result;

        // Iterate over the heap from the back
        for(auto it = countHeap.rbegin(); it != countHeap.rend() && k > 0; it++, k--) {
            result.push_back(it->second);
        }

        return result;
    }
};

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