Given an array of strings words and an integer k, return the k most frequent strings.
Return the answer sorted by the frequency from highest to lowest. Sort the words with the same frequency by their lexicographical order.
Example 1:
Input: words = ["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: words = ["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.
Constraints:
1 <= words.length <= 500
1 <= words[i].length <= 10
words[i] consists of lowercase English letters.
k is in the range [1, The number of unique words[i]]
Follow-up: Could you solve it in O(n log(k)) time and O(n) extra space?
Solutions
Solution 1: Hash Table + Sorting
We can use a hash table $\textit{cnt}$ to record the frequency of each word. Then, we sort the key-value pairs in the hash table by value, and if the values are the same, we sort by key.
Finally, we take the first $k$ keys.
The time complexity is $O(n \times \log n)$, and the space complexity is $O(n)$. Here, $n$ is the number of words.