347. Top K Frequent Elements

Top K Frequent Elements

原題介紹

Given a non-empty array of integers, return the k most frequent elements.

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note:
You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size.
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給出一個非空整數列表，返回出現頻次最大的k個元素

解題思路

O(n)

1. hash，得到<元素，頻次>鍵值對
2. 因爲頻次小於n，建散列表，即新建大小爲n+1的數組，數組下標爲頻次，數組內容爲有相同頻次的鍵值list，對散列表按下標由大到小遍歷，找出k個鍵值

ref:https://leetcode.com/discuss/100581/java-o-n-solution-bucket-sort

O(n*log(n-k))

1. hash map
2. 新建大小爲n-k的最大堆，遍歷<元素, 頻次>，頻次大於堆頂的保存到返回列表，頻次小於堆頂的入堆，最後最小堆保存的是出現頻次最低的n-k個，被“捨棄”的元素則是出現頻次最大的k個

ref:https://leetcode.com/discuss/100562/o-log-k-unordered_map-and-priority_queue-maxheap-solution