LeetCode347. Top K Frequent Elements

一、题目

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]
Example 2:

Input: nums = [1], k = 1
Output: [1]

Constraints:

1 <= nums.length <= 105
-104 <= nums[i] <= 104
k is in the range [1, the number of unique elements in the array].
It is guaranteed that the answer is unique.

Follow up: Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size.

二、题解

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        unordered_map<int,int> map;
        priority_queue<pair<int,int>> q;
        for(auto x:nums){
            map[x]++;
        }
        for(auto it = map.begin();it != map.end();it++){
            q.push({it->second,it->first});
        }
        vector<int> res;
        while(!q.empty() && k){
            res.push_back(q.top().second);
            q.pop();
            k--;
        }
        return res;
    }
};