实现嵌套链表

程序设计里面作业的一道题，感觉我们的助教师兄出的很好，考到了很多指针的知识

题干如下

You guys must have seen nested list (嵌套链表) such as [1, 2, 3, [4, 5, [6, 7]], 8, [9, 0]].

So your task is to improve DouList with nested feature. You can directly copy the codes submitted before and make changes on it. But pay attention to something which had been changed in header file.

 DouList

NOTICE 1:

front() / back() now returns a DouList
for example : a = [1, 2, 3, [4, 5, [6, 7]], 8, [9, 0]]
a.front() returns [1]
a.back() returns [9, 0]

NOTICE 2:

make sure the codes in the first version of DouListTest.cpp will still work well as before, consider how to make codes compatible.
(保持向下兼容)

/*头文件DouList.h*/

    #ifndef SSCPP2014_DOULIST_A_H
    #define SSCPP2014