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.

class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k) {
unordered_map<int, int> m;
for (int n : nums){
m
++;
}
priority_queue<pair<int, int>> pq;
for (auto n:m){
pq.push(make_pair(n.second, n.first));
}
vector<int> res(k);
for (int i = 0; i < k; i++){
res[i] = pq.top().second;
pq.pop();
}
return res;
}
};