分治法两两合并，才没有超时

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):        def mergeKLists(self, lists):        """        :type lists: List[ListNode]        :rtype: ListNode        """        if len(lists)==0:   return []        return self.merge(lists,0,len(lists)-1)        def merge(self,lists,l,r):        if l