Merge k Sorted Lists

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: []

Example 3:

Input:  lists = [[]]
Output: []

Constraints:

• k == lists.length
• 0 <= k <= 10⁴
• 0 <= lists[i].length <= 500
• -10⁴ <= lists[i][j] <= 10⁴
• Each list is sorted in ascending order
• Total nodes ≤ 10⁴