Merge K Sorted Lists

Problem Merge k sorted linked lists into one sorted list (analogous to merging already-sorted results streaming from k shards).

Input / Output

• Input: an array of k sorted list heads.
• Output: a single merged sorted list.

Constraints

• 0 ≤ k ≤ 10^4; let N be the total number of nodes.
• Handle empty lists and an empty input array.

Example

• [[1,4,5],[1,3,4],[2,6]] → [1,1,2,3,4,4,5,6].