老师，第347题按照你的提示，我提交了如下代码，但好像代码没有变得更简洁，请问如何更改进呢？

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        vector<int> res;
        //(元素，频率)
        unordered_map<int,int> freq;
        unordered_map<int,int>::iterator iter;
        for(int num:nums)
            freq[num]++;
        int n = freq.size();
        //(频率，元素)
        priority_queue<pair<int,int>> pq;
        for(iter=freq.begin();iter!=freq.end();iter++){
            if(pq.size() == n-k) {
                if(n-k!=0 && iter->second < pq.top().first) {
                    pq.pop();
                    pq.push(make_pair(iter->second,iter->first));
                }
            }
            else
                pq.push(make_pair(iter->second,iter->first));
        }
        while(!pq.empty()) {
            freq.erase(pq.top().second);
            pq.pop();
        }
        for (auto k:freq)
            res.push_back(k.first);
        return res;
    }
};