692. Top K Frequent Words

692. Top K Frequent Words

Given a non-empty list of words, return the k most 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 = 2Output: ["i", "love"]Explanation: "i" and "love" are the two most frequent words. Note that "i" comes before "love" due to a lower

Example 2:

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

Note: You may assume k is always valid, 1 ≤ k ≤ number of unique elements. Input words contain only lowercase letters. Follow up: Try to solve it in O(n log k) time and O(n) extra space.

思路： Intuition and Algorithm

Count the frequency of each word, and sort the words with a custom ordering relation that uses these frequencies. Then take the best k of them.

class Solution { public List topKFrequent(String[] words, int k) { Map count = new HashMap(); for (String word: words) { count.put(word, count.getOrDefault(word, 0) + 1); } List candidates = new ArrayList(count.keySet()); Collections.sort(candidates, (w1, w2) -> count.get(w1) != count.get(w2) ? count.get(w2) - count.get(w1) : w1.compareTo(w2)); return candidates.subList(0, k); }}

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