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.