Solution to Leetcode 23 Merge k Sorted Lists

soln_lc23.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]:
        # credit: https://www.youtube.com/watch?v=q5a5OiGbT6Q
        if not lists or len(lists) == 0:
            return None 
        while len(lists) > 1:
            res = []
            for i in range(0, len(lists), 2):
                list1 = lists[i]
                list2 = lists[i+1] if i < len(lists) - 1 else None
                res.append(self.mergeTwoLists(list1, list2))
            lists = res
        return lists[0]
        
    def mergeTwoLists(self, list1, list2):
        dummy = ListNode()
        tail = dummy
        while list1 and list2:
            if list1.val < list2.val:
                tail.next = list1
                list1 = list1.next
            else:
                tail.next = list2
                list2 = list2.next
            tail = tail.next
        if list1:
            tail.next = list1
        elif list2:
            tail.next = list2
        return dummy.next