347. Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

Example 1:

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

Example 2:

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

Note:

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
• Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

求出现频率最多的 k 个数

统计每个数出现的频率，对频率进行桶排序

C++：

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        unordered_map<int,int> map ;
        for(int num : nums){
            map[num]++ ;
        }
        vector<vector<int> > buckets(nums.size() + 1) ;
        for(auto m : map){
            buckets[m.second].push_back(m.first) ;
        }
        vector<int> res ;
        for(int i = buckets.size() - 1 ; i >= 0 && res.size() < k ; i--){
            for(int j : buckets[i]){
                res.push_back(j) ;
                if (res.size() == k)
                    break ;
            }
        }
        return res ;
    }
};