23. 合并K个排序链表

https://leetcode-cn.com/problems/merge-k-sorted-lists/

合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。

示例:

输入:
[
  1->4->5,
  1->3->4,
  2->6
]
输出: 1->1->2->3->4->4->5->6

备注

• 归并排序

# 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 lists
        return self.merge(lists, 0, len(lists)-1)

    def merge(self, lists, left, right):
        if left == right:
            return lists[left]
        mid = left + (right-left) / 2
        l1 = self.merge(lists, left, mid)
        l2 = self.merge(lists, mid+1, right)
        return self.merge_two_lists(l1, l2)

    def merge_two_lists(self, l1, l2):
        pre = ListNode(-1)
        cur = pre
        while l1 and l2:
            if l1.val <= l2.val:
                cur.next = l1
                l1 = l1.next
            else:
                cur.next = l2
                l2 = l2.next

            cur = cur.next
        cur.next = l1 or l2
        return pre.next