Top K Frequent Elements.cpp

/*
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.
*/

class Solution
{
public:
  vector<int> topKFrequent(vector<int> &nums, int k)
  {
    unordered_map<int, int> m;
    for (auto &i : nums)
      m[i]++;

    priority_queue<pair<int, int>> pq;

    for (auto it = m.begin(); it != m.end(); ++it)
    {
      pq.push({it->second, it->first});
    }

    vector<int> ans;
    while (k-- > 0)
    {
      ans.push_back(pq.top().second);
      pq.pop();
    }
    return ans;
  }
};