Merge K Sorted Lists

Write merge_k_lists(lists). lists is a Python list of linked-list heads (ListNode or None), each sorted in non-decreasing order. Return the head of a single sorted linked list made by splicing together all the original nodes (don't allocate new value nodes; a dummy head is fine).

lists = [ 1 -> 5 -> 6,   2 -> 3,   None,   0 -> 5 ]
result:  0 -> 1 -> 2 -> 3 -> 5 -> 5 -> 6

lists = []        result: None
lists = [None]    result: None

With N total nodes and k lists, aim for O(N log k). Merging the lists into an accumulator one at a time is O(N·k) and too slow for the tests (up to 40,000 lists and 120,000 nodes).