Merge K Sorted Lists

Statement

Given an array of $k$ sorted linked lists, your task is to merge them into a single sorted linked list and return the head of this linked list.

Constraints:

• $k =$ lists.length
• $0 \leq k \leq 10^3$
• $0 \leq$ lists[i].length $\leq 500$
• $-10^3 \leq$ lists[i][j] $\leq 10^3$
• Each lists[i] is sorted in ascending order.
• The sum of all lists[i].length will not exceed $10^3$.

Example