Top K Words

Given a non-empty list of words, return thekmost frequent elements.

Your answer should be sorted by frequency from highest to lowest. If two words have the same frequency, then the word with the lower alphabetical order comes first.

Example 1:

Input: ["i", "love", "leetcode", "i", "love", "coding"], k = 2

Output: ["i", "love"]

Explanation:
"i" and "love" are the two most frequent words.
Note that "i" comes before "love" due to a lower alphabetical order.

Example 2:

Input: ["the", "day", "is", "sunny", "the", "the", "the", "sunny", "is", "is"], k = 4

Output: ["the", "is", "sunny", "day"]

Explanation:
"the", "is", "sunny" and "day" are the four most frequent words,
with the number of occurrence being 4, 3, 2 and 1 respectively.

Code

class Solution {

    class Node {
        String key;
        int value;
        public Node(String key, int value) {
            this.key = key;
            this.value = value;
        }
    }

    public List<String> topKFrequent(String[] words, int k) {
        List<String> res = new LinkedList<>();
        Map<String, Integer> map = new HashMap<>();
        for (String word : words) {
            map.put(word, map.getOrDefault(word, 0) + 1);
        }

        PriorityQueue<Node> pq = new PriorityQueue<>(k,
            (a, b) -> a.value == b.value ? b.key.compareTo(a.key) : a.value - b.value
        );

        for (String word : map.keySet()) {
            pq.offer(new Node(word, map.get(word)));
            if (pq.size() > k) {
                pq.poll();
            }
        }

        while (!pq.isEmpty()) {
            res.add(0, pq.poll().key);
        }

        return res;
    }
}