public class Solution
extends Object
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`.