# # 【LeetCode】692. Top K Frequent Words 解题报告（Python）

## # 题目描述：

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 = 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.
``````

Note:

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

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

## # 解题方法

``````class Solution(object):
def topKFrequent(self, words, k):
"""
:type words: List[str]
:type k: int
:rtype: List[str]
"""
count = collections.Counter(words)
def compare(x, y):
if x[1] == y[1]:
return cmp(x[0], y[0])
else:
return -cmp(x[1], y[1])
return [x[0] for x in sorted(count.items(), cmp = compare)[:k]]
``````

``````class Solution(object):
def topKFrequent(self, words, k):
"""
:type words: List[str]
:type k: int
:rtype: List[str]
"""
count = collections.Counter(words)
candidates = count.keys()
candidates.sort(key = lambda w: (-count[w], w))
return candidates[:k]
``````

`heapq.heapify(heap)`能在线性时间内，把一个列表转成堆。

`heapq.heappop(heap)`能直接弹出堆的堆顶。

`heappush(heap,5)`向堆中添加元素。

python中的堆默认是小根堆，如果想使用大根堆，在添加元素的时候使用负号即可。

``````class Solution(object):
def topKFrequent(self, words, k):
"""
:type words: List[str]
:type k: int
:rtype: List[str]
"""
count = collections.Counter(words)
heap = [(-freq, word) for word, freq in count.items()]
heapq.heapify(heap)
return [heapq.heappop(heap)[1] for _ in range(k)]
``````

2018 年 3 月 14 日