问题
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Worst-case O(n) algorithm for doing k-selection
Given the following question :
In an integer array with N elements , find the minimum k elements (k << N)
You can assume that N is a large number.
I'm thinking about a minimum heap , anyone has a better solution ?
Regards
回答1:
If K << N, min heap is good enough because creation of heap is O(n), and if K << N selecting first K items is at most O(N), otherwise you could use selection algorithm to find Kth smallest element in O(n) then select numbers which are smaller than found item. (Sure if some numbers are equal to Kth element select till fill K items).
回答2:
What about this:
Sort the array (quicksort or heapsort are great for integer arrays), and iterate to k
回答3:
I think you can do this one in O(N*log(K)). Pseudocode:
haz array[N]
haz output[k] (itz a list)
i iteratez on array with array[N] az element:
i insertz element into output (i maintainz strict ordering)
i removez largest element of output when output size iz bigger than k
Requires:
- N list removals from end (N * O(1))
- at most N sort-maintaining list inserts (N * O(log(listsize)))
list's size is bounded by K
Thus, O(N * log(K)) time.
来源：https://stackoverflow.com/questions/12057195/in-an-integer-array-with-n-elements-find-the-minimum-k-elements