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

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note:

• class Solution {
  public:
      vector<int> topKFrequent(vector<int>& nums, int k) {
          unordered_map<int,int> mp;
          for(auto &i : nums)
              mp[i]
   
  ++;
          priority_queue<pair<int,int>> pq;
          for(auto &i : mp) 
              pq.push(make_pair(i.second, i.first));
          vector<int> res;
          while(k-- > 0 && !pq.empty()) {
              res.push_back(pq.top().second);
              pq.pop();
          }
          
   
  return res;
      }
  };

  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.

[leetcode]Heap-347. Top K Frequent Elements