Merge k Sorted Lists - Leetcode 23 - Python

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: []

class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        if not lists or len(lists) == 0 :
            return None
        while len(lists) > 1 :
            ml = []
            
            for i in range(0, len(lists), 2):
                l1 = lists[i]
                l2 = lists[i+1] if (i+1) <len(lists) else None
                ml.append(self.mergeTwoLists(l1,l2))
            lists = ml
        return lists[0]
    
    def mergeTwoLists(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        d = ListNode()
        p = d
        
        while l1 and l2 :
            if l1.val < l2.val :
                p.next = l1
                l1 = l1.next 
            else :
                p.next = l2
                l2 = l2.next
            
            p = p.next
            
            
        if l1 :
            p.next = l1
        else :
            p.next = l2
            
        return d.next

Explaination :