Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity

import heapq
class Solution(object):
    def merge_klists(self, lists):
        def travel(head):
            while head is not None:
                yield head.val, head
                head = head.next
        dummy = cursor = ListNode(None)
        for _, node in heapq.merge(*map(travel, lists)):
            cursor.next = node
            cursor = cursor.next
        return dummy.next