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

挺简单的。。链表不适合排序，那就从里头拿出来放数组排序，排完放回去就好了。。时间复杂度就看次数最高的，while循环次数，根据链表长度，所以最好的时间复杂度是N最差是n^2

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        num_list = []
        if lists == []:
            return []
        else:
            for i in lists:
                while i:
                    num_list.append(i.val)
                    i = i.next

        num_list.sort()
        head = ListNode(0)
        first = head
        for i in num_list:
            head.next = ListNode(i)
            head = head.next

        return first.next