Merging K sorted list using divide and conquer technique.

merge_k.py

# 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]:
        if not lists:
            return None
        
        s = len(lists)
        
        if s == 1:
            return lists[0]
        
        mid = s // 2
        
        l = self.mergeKLists(lists[:mid])
        r = self.mergeKLists(lists[mid:])
        
        return self.merge(l, r) 
        
    def merge(self, l: Optional[ListNode], r: Optional[ListNode]) -> Optional[ListNode]:
        head = tail = ListNode(0)

        while l and r:
            if l.val <= r.val:
                tail.next = l
                l = l.next
            else:
                tail.next = r
                r = r.next

            tail = tail.next

        tail.next = l or r
        
        return head.next