Question

I am thinking about different solutions for one problem. Assume we have K sorted linked lists and we are merging them into one. All these lists together have N elements.

The well known solution is to use priority queue and pop / push first elements from every lists and I can understand why it takes O(N log K) time.

But let’s take a look at another approach. Suppose we have some MERGE_LISTS(LIST1, LIST2) procedure, that merges two sorted lists and it would take O(T1 + T2) time, where T1 and T2 stand for LIST1 and LIST2 sizes.

What we do now generally means pairing these lists and merging them pair-by-pair (if the number is odd, last list, for example, could be ignored at first steps). This generally means we have to make the following “tree” of merge operations:

N1, N2, N3... stand for LIST1, LIST2, LIST3 sizes

• O(N1 + N2) + O(N3 + N4) + O(N5 + N6) + ...
• O(N1 + N2 + N3 + N4) + O(N5 + N6 + N7 + N8) + ...
• O(N1 + N2 + N3 + N4 + .... + NK)

It looks obvious that there will be log(K) of these rows, each of them implementing O(N) operations, so time for MERGE(LIST1, LIST2, ... , LISTK) operation would actually equal O(N log K).

My friend told me (two days ago) it would take O(K N) time. So, the question is – did I f%ck up somewhere or is he actually wrong about this? And if I am right, why this ‘divide&conquer’ approach can’t be used instead of priority queue approach?

Answer 1

If you have a small number of lists to merge, this pairwise scheme is likely to be faster than a priority queue method because you have extremely few operations per merge: basically just one compare and two pointer reassignments per item (to shift into a new singly-linked list). As you’ve shown, it is O(N log K) (log K steps handling N items each).

But the best priority queue algorithms are, I believe, O(sqrt(log K)) or O(log log U) for insert and remove (where U is the number of possible different priorities)–if you can prioritize with a value instead of having to use a compare–so if you are merging items that can be given e.g. integer priorities, and K is large, then you’re better off with a priority queue.