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 alphabetical order.

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 respectively.

 Note:

1. You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
2. Input words contain only lowercase letters.

Follow up:

1. Try to solve it in O(n log k) time and O(n) extra space.

解法：hashmap + priority queue

1 class Solution { 2     public List
     
       topKFrequent(String[] words, int k) { 3  4         List
      
        result = new LinkedList<>(); 5         Map
       
         map = new HashMap<>(); 6         for (int i = 0; i < words.length; i++) { 7             map.put(words[i], map.getOrDefault(words[i], 0) + 1); 8         } 9 10         PriorityQueue
        
         
          > pq = new PriorityQueue<>(11                 (a, b) -> a.getValue() == b.getValue() ? b.getKey().compareTo(a.getKey()) : a.getValue() - b.getValue());12 13         for (Map.Entry
          
            entry : map.entrySet()) {14 pq.offer(entry);15 if (pq.size() > k) {16 pq.poll();17 }18 }19 20 while (!pq.isEmpty()) {21 result.add(0, pq.poll().getKey());22 }23 24 return result;25 }26 }
          
         
        
       
      
     

 其实这题还可以用quick sort 来解，复杂度更低。