LeetCode:Top K Frequent Elements

Top K Frequent Elements

Total Accepted: 5045  Total Submissions: 11511  Difficulty: Medium

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

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思路：桶排序

有6个数：1 1 1 2 2 3

key:0 1 2 3 4 5 6

val:0 3 2 1 0 0 0

其中：val代表key出现的频数。

根据频数大小：

key:1 2 3

val:3 2 1

其中key代表频数，val代表频数对应的数

java code:

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