23. 合并多个排序链表：https://leetcode-cn.com/problems/merge-k-sorted-lists/

这道题其实比较简单，关键是掌握堆和分治法， 写分治法的时候，涉及到递归，这块是我的一个弱项，一开始没写出来，分治法模板可以参考一下：

class Solution(object):
    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        return self.helper(0, len(lists), lists)
    # 分治法合并
    def helper(self, start, end, lists):
        if start == end:
            return lists[start]
        mid = start + (end - start) / 2
        left = self.helper(start, mid, lists)
        right = self.helper(mid+1, end, lists)
        return self.mergeTwoSortLists(left, right)

    def mergeTwoSortLists(self, list1, list2):
        '''
        @description: 合并两个排序链表
        @param {type} 
                list1: <ListNode>
                list2: <ListNode>
        @return: 
                result: <ListNode, 排序好的合并链表>
        @author: zhangzhen20
        '''  
        dummy = listNode(-1)
        cur = dummy
        while list1 and list2:
            if list1.val < list2.val:
                cur.next = list1
                list1 = list1.next
            else:
                cur.next = list2
                list2 = list2.next

        if list1 is None:
            cur.next = list2
        else:
            cur.next = list1

        return dummy.next