Java [Leetcode 347]Top K Frequent Elements

题目描述：

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.

解题思路：

使用HashMap计数

代码如下：

public class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {

    	Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    	List<Integer>[] freqArray = new List[nums.length + 1];

        for(int num : nums){
        	map.put(num, map.getOrDefault(num, 0) + 1);
        }

        for(int key : map.keySet()){
        	int freq = map.get(key);
        	if(freqArray[freq] == null)
        		freqArray[freq] = new ArrayList<Integer> ();
        	freqArray[freq].add(key);
        }

        List<Integer> res = new ArrayList<>();

        for(int i = freqArray.length - 1; i >= 0 & res.size() < k; i--){
        	if(freqArray[i] != null)
        		res.addAll(freqArray[i]);
        }

        return res;

    }
}