Merge k sorted linked lists

You are given k sorted singly linked lists (each ordered ascending). Splice them into one sorted linked list and return its head.

1 -> 4 -> 5
1 -> 3 -> 4      ->   1 -> 1 -> 2 -> 3 -> 4 -> 4 -> 5 -> 6
2 -> 6

Target $O(N \log k)$ total time for $N$ nodes, and $O(k)$ extra memory.