Problem
Merge k sorted linked lists into a single sorted list.
Input / Output
- Input: an array of k sorted linked lists (some may be empty).
- Output: one merged sorted linked list containing every node.
Constraints
- 0 ≤ k ≤ 10^4; the total number of nodes N can be large.
- Each list is individually sorted in non-decreasing order.
Example
- [[1,4,5],[1,3,4],[2,6]] → [1,1,2,3,4,4,5,6]
- [] → []; [[]] → []