347. 前 K 个高频元素

class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        if (nums == null || nums.length == 0 || k < 1) return new int[0];
        Map<Integer, Element> map = new HashMap();
        PriorityQueue<Element> pq = new PriorityQueue<>((e1, e2) -> e2.freq - e1.freq);
        for (int n : nums) {
            if (map.containsKey(n)) {
                map.get(n).incr();
            } else {
                map.put(n, new Element(n, 1));
            }
        }
        for (Element e : map.values()) {
            pq.add(e);
        }
        int[] res = new int[k];
        for (int i = 0; i < k; i++) {
            res[i] = pq.poll().val;
        }
        return res;
    }
    public class Element {
        public int val;
        public int freq;
        public Element(int val, int freq) {
            this.val = val;
            this.freq = freq;
        }
        public void incr() {
            freq++;
        }
    }
}

使用**大顶堆**优化下：

class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        if (nums == null || nums.length == 0 || k < 1) return new int[0];
        Map<Integer, Integer> map = new HashMap();
        PriorityQueue<Map.Entry<Integer, Integer>> pq = new PriorityQueue<>((e1, e2) -> e2.getValue() - e1.getValue());
        for (int n : nums) {
            map.put(n, map.getOrDefault(n, 0) + 1);
        }
        for (Map.Entry<Integer, Integer> e : map.entrySet()) {
            pq.add(e);
        }
        int[] res = new int[k];
        for (int i = 0; i < k; i++) {
            res[i] = pq.poll().getKey();
        }
        return res;
    }
}

使用小顶堆

class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        if (nums == null || nums.length == 0 || k < 1) return new int[0];
        Map<Integer, Integer> map = new HashMap();
        PriorityQueue<Map.Entry<Integer, Integer>> pq = new PriorityQueue<>((e1, e2) -> e1.getValue() - e2.getValue());
        for (int n : nums) {
            map.put(n, map.getOrDefault(n, 0) + 1);
        }
        for (Map.Entry<Integer, Integer> e : map.entrySet()) {
            pq.add(e);
            if (pq.size() == k + 1) pq.remove();
        }
        int[] res = new int[k];
        for (int i = 0; i < k; i++) {
            res[i] = pq.poll().getKey();
        }
        return res;
    }
}