LeetCode "Top K Frequent Elements"

A typical solution is heap based - "top K". Complexity is O(nlgk).

typedef pair<int, unsigned> Rec;
struct Comp
{
bool operator()(const Rec &r1, const Rec &r2)
{
return r1.second > r2.second;
}
};
class Solution {

public:
vector<int> topKFrequent(vector<int>& nums, int k) {
unordered_map<int, unsigned> hm;
for(auto&v : nums) hm[v]++;

priority_queue<Rec, vector<Rec>, Comp> q;
for(auto &kv : hm)
{
Rec r(kv.first, kv.second);
q.push(r);
if(q.size() > k)    q.pop();
}

vector<int> ret;
while(!q.empty())
{
ret.push_back(q.top().first);
q.pop();
}
return ret;
}
};

There is a O(n) one indeed - bucketing the frequencies.
https://leetcode.com/discuss/100636/c-o-nlogk-and-o-n-solutions