代写数据结构里面的单链表实现，稍微麻烦一点的是，单链表排序需要用递归来实现。
Requirement
Using C++ Class and pointers, implement Single-Linked List of integers. You
are not allowed to use STL. Consider the following for your linked list
implementation:
- Update the List class implementations in the lecture slides so that the List does not have any duplicate value.
- Also make the necessary updates to the List class to maintain a sorted list (i.e. the Nodes are in increasing order of data values). In particular, replace all the insert methods, and replace them with insert(int x), which inserts x in the right position so that the List remains sorted.
- Implement DeleteMid() to delete the middle element (when the list contains an odd number of elements) or middle two elements (when the list contains an even number of elements).
- Using recursion, implement Ascend() and Descend() to display the entire list in ascending and descending orders, respectively.
Sample Input
3 // Total no of input sets
5 10 4 4 // Input set 1
1 5 3 5 9 -1 7 // Input set 2
5 4 3 2 -999 // Input set 3
Sample Output (for Input set 1)
Sorted List: 4 5 10
After Deleting Middle Element:
List: 4 10
Display List in Ascending Order:
4 10
Display List in Descending Order:
10 4
and so on for other sample input sets 2 and 3
