347. Top K Frequent Elements

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

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2:

Input: nums = [1], k = 1
Output: [1]

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.

• // use maxHeap. Put entry into maxHeap so we can always poll a number with largest frequency
  class Solution {
      public List<Integer> topKFrequent(int[] nums, int k) {
          Map<Integer, Integer> map = new HashMap<>();
          for(int n: nums){
              // map.put(n, map.getOrDefault(n,0)+1);
              if(!map.containsKey(n)){
                  map.put(n, 1);
              }else{
                  map.put(n, map.get(n) + 1);
              }
          }
             
          PriorityQueue<Map.Entry<Integer, Integer>> maxHeap = 
                           new PriorityQueue<>((a,b)->(b.getValue()-a.getValue()));
          for(Map.Entry<Integer,Integer> entry: map.entrySet()){
              maxHeap.add(entry);
          }
          
          List<Integer> res = new ArrayList<>();
          while(res.size()<k){
              Map.Entry<Integer, Integer> entry = maxHeap.poll();
              res.add(entry.getKey());
          }
          return res;
      }
  }