Merge k Sorted Lists

Given k sorted linked lists, merge them into one sorted linked list and return its head.

Examples
Input: [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Hints
Related Problems

Merge k Sorted Lists

Given `k` sorted linked lists, merge them into one sorted linked list and return its head.