Code golf: combining multiple sorted lists into a single sorted list

Question

Implement an algorithm to merge an arbitrary number of sorted lists into one sorted list. The aim is to create the smallest working programme, in whatever language you like.

Answer 1

(all other solutions are O(N) (for the input provided))

If we let N be the number of elements in the output and k the number of input lists, then you can't do faster than O(N log k) -- suppose that each list was only a single element, and you'd have faster-than-O(N log N) comparison-based sorting.

Those I've looked at look more like they're O(N*k).

You can fairly easily get down to O(N log k) time: just put the lists in a heap. This is one of the ways to do I/O-efficient sorting (you can generalize quicksort and heaps/heapsort as well).

[no code, just commentary]