leetcode 347.Top K Frequent Elements

链接:https://leetcode.com/problems/top-k-frequent-elements/

题目:

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.

题目大意:给你一个数组num和一个k值，找出在num数组中出现最频繁的前K个数。

思路:比较简单，利用hashMap统计出现次数，利用排序或者优先队列都可以进行最后的取出。题目没有对空间复杂度有要求，所以我们应该可以使用这种方法。代码如下:

public class Solution {
public List<Integer> topKFrequent(int[] nums, int k) {
//利用hashMap，和优先级队列。
List<Integer> res=new ArrayList<Integer>();
Map<Integer,Integer> map=new HashMap<Integer,Integer>();
if(k>nums.length||k<1){
return res;
}
for(int num:nums){
if(map.containsKey(num)){
map.put(num,map.get(num)+1);
}else{
map.put(num,0);
}
}
ArrayList<Map.Entry<Integer,Integer>> list = new ArrayList<Map.Entry<Integer, Integer>>(map.entrySet());
Collections.sort(list, new Comparator<Map.Entry<Integer, Integer>>() {
@Override
public int compare(Map.Entry<Integer, Integer> o1, Map.Entry<Integer, Integer> o2) {
return o2.getValue() - o1.getValue();
}
});
int count = 0;
while (count < k)
res.add(list.get(count++).getKey());
return res;
}
}