You are given an array of `k` linked lists, each sorted in ascending order. Merge all the linked lists into one sorted linked list and return it.
**Example 1:**
```
Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
```
**Example 2:**
```
Input: lists = []
Output: []
```