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.
题意：
给定数组，求其中出现频率最高的K个元素。
Solution: PriorityQueue
扫数组，将数组每个元素和其对应的出现频率存到Map里
扫Map，将Map里的元素放入maxHeap
从maxHeap里poll出K个元素即可
code
class Solution {
public List<Integer> topKFrequent(int[] nums, int k) {
Map<Integer, Integer> numFreqs = new HashMap();
for(int num : nums){
numFreqs.put(num, numFreqs.containsKey(num) ? numFreqs.get(num) + 1 : 1);
}
PriorityQueue <Map.Entry<Integer, Integer>> maxHeap = new PriorityQueue<>((entry1, entry2) -> entry2.getValue() - entry1.getValue());
for(Map.Entry<Integer, Integer> entry : numFreqs.entrySet()){
maxHeap.add(entry);
}
List<Integer> result = new ArrayList<>();
while(!maxHeap.isEmpty() && result.size() < k){
result.add(0, maxHeap.poll().getKey());
}
return result;
}
}