347. Top K Frequent Elements [leetcode]

法 一：

hashmap + vector

时间：O（n）

class Solution {public:    vector<int> topKFrequent(vector<int>& nums, int k) {        vector<int> res;        if (k < 1) return res;        if (k >= nums.size()) return nums;                vector<vector<int>> count(nums.size() + 1, vector<int>());        unordered_map<int, int> countList;        for (auto i : nums) countList[i]++;        for (unordered_map<int, int>::iterator it = countList.begin(); it != countList.end(); it++)             count[it->second].push_back(it->first);        for (int i = nums.size(); i >= 1 && k > 0; i--) {            if (!count[i].empty()) {                for (auto j : count[i]) {                    res.push_back(j);                    k--;                }             }        }                   return res;    }};

法二：

hashmap + priority_queue

时间：O（nlongn）

class Solution {public:    vector<int> topKFrequent(vector<int>& nums, int k) {        vector<int> res;        if (k < 1) return res;        if (k >= nums.size()) return nums;                      unordered_map<int, int> countList;        for (auto i : nums) countList[i]++;        priority_queue<pair<int,int>> q;        int n = 1;        for (unordered_map<int, int>::iterator it = countList.begin(); it != countList.end(); it++, n++) {            q.push(make_pair(it->second, it->first));            if (n > countList.size() - k) {                res.push_back(q.top().second);                q.pop() ;            }        }                               return res;    }};