23-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 None
        while len(lists) > 1:
            m_list = []

            for i in range(0, len(lists), 2):
                h1 = lists[i]
                h2 = lists[i + 1] if (i + 1) < len(lists) else None
                
                m_list.append(self.mergeTwoLists(h1, h2))
            lists = m_list
        return lists[0]


    def mergeTwoLists(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
        if list1:
            tail.next = list1
        elif list2:
            tail.next = list2
        return dummy.next