Merge k Sorted Lists

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 linked 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` will not exceed `10^4`.