leetcode 347前k个高频元素

 

 

通过hash map遍历一遍存储出现的次数，通过小顶堆存储k个元素

//设想利用hash map来存储每个元素的个数；采用小顶堆存储k个元素；timeO(n+klogk)spaceO(n+k)

class Solution {
public:
    
    
    vector<int> topKFrequent(vector<int>& nums, int k) {
        if(nums.size()==0) return{};
        vector<int> res;
        unordered_map<int,int> m;
        for(int num:nums){
            m[num]++;
        }
        struct cmp{
            bool operator()(pair<int,int> a,pair<int,int> b) {return a.second>b.second;}//大于才是小顶堆
        };
        priority_queue<pair<int,int>,vector<pair<int,int> >,cmp> q;
        for(auto it:m){
            int num=it.first;
            //cout<<it.first<<",  "<<it.second<<endl;
            if(q.size()<k){
                q.push(it);continue;
            }
            //cout<<it.first<<","<<it.second<<endl;
            //cout<<q.top().first<<","<<q.top().second<<endl;
            if(it.second>q.top().second){
                q.pop();q.push(it);
            }
        }
        while(!q.empty()){
            res.push_back(q.top().first);
            q.pop();
        }
        return res;
    }
};