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TopKFrequent.java
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63 lines (51 loc) · 1.73 KB
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package com.lga.algorithm.tag.homework.Week_02;
import org.junit.Assert;
import org.junit.Test;
import java.lang.reflect.Array;
import java.util.*;
/**
* 347. 前 K 个高频元素
* https://leetcode-cn.com/problems/top-k-frequent-elements/
*/
public class TopKFrequent {
/**
*构建一个大小为K的小顶堆
* @param nums
* @param k
* @return
*/
public int[] topKFrequent_minHeap(int[] nums, int k) {
Map<Integer, Integer> map = new HashMap<>();
int[] ans = new int[k];
//将nums 中的数字和出现的频率构建一个哈希表
for (int num : nums) {
map.put(num, map.get(num) != null? map.get(num) + 1 : 1);
}
//构建一个小顶堆
PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue(k, new Comparator<Map.Entry<Integer,Integer>>() {
@Override
public int compare(Map.Entry<Integer, Integer> o1, Map.Entry<Integer, Integer> o2) {
return o1.getValue()-o2.getValue();
}
});
for (Map.Entry<Integer, Integer> mapEntry : map.entrySet()) {
if (minHeap.size() < k) {
minHeap.add(mapEntry);
}else{
if (minHeap.peek().getValue() < mapEntry.getValue()) {
minHeap.poll();
minHeap.add(mapEntry);
}
}
}
//获取小顶堆中的值，
while (!minHeap.isEmpty() && k-->0) {
ans[k] = minHeap.poll().getKey();
}
return ans;
}
@Test
public void test_topKFrequent_minHeap() {
Assert.assertTrue(Arrays.equals(new int[]{2,-1},topKFrequent_minHeap(new int[]{4,1,-1,2,-1,2,3},2)));
}
}