Merge K Sorted Lists

The problem. You're given an array of k linked lists, each sorted in ascending order. Merge them all into one sorted linked list and return its head.

Example

lists = [[1,4,5], [1,3,4], [2,6]]   ->  [1,1,2,3,4,4,5,6]
lists = []                          ->  []
lists = [[]]                        ->  []

Below, N is the total number of nodes in all the lists, and k is the number of lists.