给你一个链表数组，每个链表都已经按升序排列。
请你将所有链表合并到一个升序链表中，返回合并后的链表。
解题思路
分治加两个链表合并，或者用根堆，根堆的初始化方法很值得背诵，还涉及lambda的用法
AC代码
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
ListNode.__lt__ = lambda a, b: a.val < b.valclass Solution:def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:h = [head for head in lists if head]dummyhead = ListNode()indexhead = dummyheadheapify(h)while h:node = heappop(h)if node.next:heappush(h, node.next)indexhead.next = nodeindexhead = indexhead.nextreturn dummyhead.next
)