LeetCode Top K Frequent Elements

题意：给出一个数组，求出前k个出现频率最多的数

思路：首先统计每个元素的次数（用Map）,然后用堆排序只统计前k个(PriorityQueue)

代码如下：

class Solution
{class Pair implements Comparable<Pair>{int key, value;public int compareTo(Pair b){return value - b.value;}}public List<Integer> topKFrequent(int[] nums, int k){Map<Integer, Integer> map = new HashMap<Integer, Integer>();for (int i = 0; i < nums.length; i++){if (map.containsKey(nums[i])){map.put(nums[i], map.get(nums[i]) + 1);}else{map.put(nums[i], 1);}}Comparator<Pair> cmp = new Comparator<Pair>(){public int compare(Pair a, Pair b){return a.value - b.value;}};Queue<Pair> queue = new PriorityQueue<Pair>(k, cmp);for(Map.Entry<Integer, Integer> ele: map.entrySet()){Pair p = new Pair();p.key = ele.getKey(); p.value = ele.getValue();if (queue.size() < k){queue.add(p);}else{if (p.compareTo(queue.peek()) > 0){queue.poll();queue.offer(p);}}}List<Integer> res = new ArrayList<Integer>();while (!queue.isEmpty()){Pair p = queue.poll();res.add(p.key);}Collections.sort(res);return res;}}

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