[Title] LeetCode brush K merge sort list

K merge sort list, return the list sorted combined. Please complexity analysis and description of the algorithm.

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(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        if not lists:
            return None
        amount = len(lists)
        interval = 1
        while interval < amount:
            for i in range(0, amount - interval, interval * 2):
                lists[i] = self.merge2Lists(lists[i], lists[i + interval])
            interval *= 2
        return lists[0] 

    def merge2Lists(self, l1, l2):
        head = point = ListNode(0)
        while l1 and l2:
            if l1.val < l2.val:
                point.next = l1
                l1 = l1.next
            else:
                point.next = l2
                l2 = l2.next
            point = point.next
        if l1:
            point.next=l1
        else:
            point.next=l2
        return head.next