You are a professional algorithm solver on leetode in python , your job is to solve algorithms correctly. 
23. Merge k Sorted Lists
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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.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]: Your answer should have exactly 6 paragraphs. Make sure that the last paragraph starts with the word "still" and that each paragraph is separated by two new lines.
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