(Collection)347. Top K Frequent Elements

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

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note: 

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.

• public class Solution { //桶排序          
      public List<Integer> topKFrequent(int[] nums, int k) {  //也可以使用PriorityQueue
         Map<Integer,Integer> map=new HashMap<Integer,Integer>();
         List<Integer> res =new ArrayList<Integer>();
         for(int num: nums){
             map.put(num,map.getOrDefault(num,0)+1);
         }
         List<Integer>[] bucket=new List[nums.length+1];
         for(int key : map.keySet()){
             int freq=map.get(key);
             if(bucket[freq]==null){
                 bucket[freq]=new ArrayList<Integer>();
             }
             bucket[freq].add(key);
         }
         for(int i=nums.length;i>=0 && res.size()<k;i--){
            if(bucket[i]!=null)
             res.addAll(bucket[i]);
         }
         return res;
      }
  }
  

  　　

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