347. Top K Frequent Elements

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: 

• 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.

 O(n log(n-k)) unordered_map and priority_queue(maxheap) solution:

class Solution {public:    vector<int> topKFrequent(vector<int>& nums, int k) {        unordered_map<int,int> map;        for(int num : nums){            map[num]++;        }                vector<int> res;        // pair<first, second>: first is frequency,  second is number        priority_queue<pair<int,int>> pq;         for(auto it = map.begin(); it != map.end(); it++){            pq.push(make_pair(it->second, it->first));            if(pq.size() > (int)map.size() - k){                res.push_back(pq.top().second);                pq.pop();            }        }        return res;    }};