23\. Merge k Sorted Lists

Hard

You are given an array of `k` linked-lists `lists`, each linked-list is 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]

**Explanation:** The linked-lists are: [ 1->4->5, 1->3->4, 2->6 ] merging them into one sorted list: 1->1->2->3->4->4->5->6 

**Example 2:**

**Input:** lists = []

**Output:** [] 

**Example 3:**

**Input:** lists = [[]]

**Output:** [] 

**Constraints:**

*   `k == lists.length`
*   `0 <= k <= 10^4`
*   `0 <= lists[i].length <= 500`
*   `-10^4 <= lists[i][j] <= 10^4`
*   `lists[i]` is sorted in **ascending order**.
*   The sum of `lists[i].length` won't exceed `10^4`.