Given an array of k linked lists, each sorted in ascending order, merge all lists into one sorted linked list.
With [[1,4,5], [1,3,4], [2,6]], the answer is 1 → 1 → 2 → 3 → 4 → 4 → 5 → 6. You weave together all elements in sorted order.
This is a classic problem that combines linked list manipulation with efficient merging strategies.
Constraints: . Total nodes up to .