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 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`.