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

Example:

Input:

[

1->4->5,

1->3->4,

2->6]

Output: 1->1->2->3->4->4->5->6

# Definition for singly-linked list.class ListNode:    def __init__(self, x):        self.val = x        self.next = Noneclass Solution:    def mergeKLists(self, lists):        """        :type lists: List[ListNode]        :rtype: ListNode        """        if len(lists)==0:            return None        if len(lists)==1:            return lists[0]        head = self.mergeTwoLists(lists[0],lists[1])        for i in range(2,len(lists)):            head = self.mergeTwoLists(head,lists[i])        return head    def mergeTwoLists(self, l1, l2):        """        :type l1: ListNode        :type l2: ListNode        :rtype: ListNode        """        if l1 is None and l2 is None:            return l1        if l1 is None:            return l2        if l2 is None:            return l1        start = ListNode(0)        pos = start        while l1 and l2:            if l1.val