合并 K 个升序链表

class Solution:    def mergeKLists(self, lists):        if not lists: return None        if len(lists) == 1: return lists[0]        mid = len(lists) // 2        left  = self.mergeKLists(lists[:mid])   # 左半递归合成一条        right = self.mergeKLists(lists[mid:])   # 右半递归合成一条        return self.mergeTwo(left, right)       # 再合并这两条    def mergeTwo(self, a, b):                   # 合并两条有序链(LC21)        dummy = cur = ListNode(0)        while a and b:            if a.val <= b.val: cur.next, a = a, a.next            else:              cur.next, b = b, b.next            cur = cur.next        cur.next = a or b                       # 残段整条接上        return dummy.next

import heapqclass Solution:    def mergeKLists(self, lists):        h = [(node.val, i, node) for i, node in enumerate(lists) if node]        heapq.heapify(h)                 # K 个头进堆        dummy = cur = ListNode(0)        while h:            val, i, node = heapq.heappop(h)        # 弹出全局最小头            cur.next = node; cur = node            if node.next:                heapq.heappush(h, (node.next.val, i, node.next))        return dummy.next