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 <= 104`
- `0 <= lists[i].length <= 500`
- `-104 <= lists[i][j] <= 104`
- `lists[i]` is sorted in ascending order.
- The sum of `lists[i].length` will not exceed `104`.