content: import java.util.LinkedList;
  import java.util.List;
  import java.util.TreeMap;
  
  class Solution {
  
      private class Freq implements Comparable<Freq> {
  
          // 元素的值和频率
          public int e, freq;
  
          public Freq () {}
          public Freq (int e, int freq) {
              this.e = e;
              this.freq = freq;
          }
  
          @Override
          public int compareTo(Freq another) {
  
              // 最大堆，取出优先级最高的那个元素，
              // 频次最低的那个元素才是优先级最高的
  
              // 频次低的话优先级才高
              if (this.freq < another.freq)
                  return 1;
              else if (this.freq > another.freq)
                  return -1;
              else
                  return 0;
          }
      }
  
      public List<Integer> topKFrequent(int[] nums, int k) {
  
          // 先将数组中的元素放入 TreeMap中，统计每个数字出现的次数
          TreeMap<Integer, Integer> treeMap = new TreeMap<Integer, Integer>();
  
          for (int num: nums) {
              if (treeMap.containsKey(num))
                  treeMap.put(num, treeMap.get(num) + 1);
              else
                  treeMap.put(num, 1);
          }
  
          // 使用自定义的优先队列
          MyPriorityQueue<Freq> priorityQueue = new MyPriorityQueue<Freq>();
  
          // 这样的一个算法就是nlogk这样一个级别的算法
          for (int key: treeMap.keySet()) {
              // 如果还没有存够k个元素
              if (priorityQueue.getSize() < k)
                  priorityQueue.enqueue(new Freq(key, treeMap.get(key)));
              else if (treeMap.get(key) > priorityQueue.getFront().freq) {
                  priorityQueue.dequeue();
                  priorityQueue.enqueue(new Freq(key, treeMap.get(key)));
              }
          }
          LinkedList<Integer> result = new LinkedList<Integer>();
          while (!priorityQueue.isEmpty())
              result.add(priorityQueue.dequeue().e);
          return result;
      }
  
      public static void main(String[] args) {
          int[] testData = {4,1,-1,2,-1,2,3};//{3,0,1,0};
          Solution solution = new Solution();
          System.out.println(solution.topKFrequent(testData, 2));
      }
  }