0023-Merge-k-Sorted-Lists.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

        while len(lists) > 1:
            merged_lists = []
            lists_len = len(lists)
            for idx in range(0, lists_len, 2):
                l1 = lists[idx]
                next_id = idx + 1
                l2 = lists[next_id] if next_id < lists_len else None
                new_list = self.merge_two_sorted_lists(l1, l2)
                merged_lists.append(new_list)
            lists = merged_lists
        return lists[0]

    def merge_two_sorted_lists(
        self,
        list1: Optional[ListNode],
        list2: Optional[ListNode],
    ) -> Optional[ListNode]:
        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

        tail.next = list1 or list2

        return dummy.next