<p>You are given an array of <code>k</code> linked-lists <code>lists</code>, each linked-list is sorted in ascending order.</p>

<p><em>Merge all the linked-lists into one sorted linked-list and return it.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> lists = [[1,4,5],[1,3,4],[2,6]]
<strong>Output:</strong> [1,1,2,3,4,4,5,6]
<strong>Explanation:</strong> The linked-lists are:
[
  1-&gt;4-&gt;5,
  1-&gt;3-&gt;4,
  2-&gt;6
]
merging them into one sorted list:
1-&gt;1-&gt;2-&gt;3-&gt;4-&gt;4-&gt;5-&gt;6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> lists = []
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> lists = [[]]
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>k == lists.length</code></li>
	<li><code>0 &lt;= k &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= lists[i].length &lt;= 500</code></li>
	<li><code>-10<sup>4</sup> &lt;= lists[i][j] &lt;= 10<sup>4</sup></code></li>
	<li><code>lists[i]</code> is sorted in <strong>ascending order</strong>.</li>
	<li>The sum of <code>lists[i].length</code> will not exceed <code>10<sup>4</sup></code>.</li>
</ul>

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def getMiddle(self,head):
        fast,slow=head,head
        while fast.next and fast.next.next:
            slow=slow.next
            fast=fast.next.next
        return slow

    def mergeTwoLists(self, list1: ListNode, list2: ListNode) -> ListNode:
        dummy = node = ListNode()

        while list1 and list2:
            if list1.val < list2.val:
                node.next = list1
                list1 = list1.next
            else:
                node.next = list2
                list2 = list2.next
            node = node.next

        node.next = list1 or list2
        return dummy.next

    def divide(self,head):
        if not head.next:
            return head
        
        middle=self.getMiddle(head) 
        right=middle.next
        middle.next=None
        left=self.divide(head)
        right=self.divide(right)

        return self.mergeTwoLists(left,right)

    def sortList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head:
            return head

        return self.divide(head)

    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        dummy = cur = ListNode()
        while True:
            isEnded=True
            for i,l in enumerate(lists):
                if l is not None:
                    isEnded=False
                    cur.next=l
                    cur=cur.next
                    lists[i]=l.next
            if isEnded:
                return self.sortList(dummy.next)

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