Warning: You are not allowed to use:
1. any static variables
2. any variables which is not inside a function
3. malloc and free
由於實際的code有做其他檢查，因此為了讓各位方便閱讀，請參考簡化版本的function.h與main.cpp（請點連結）
You are required to implement a list.
You have to enable C++11 in this problem. (e.g. "g++ -std=c++11 ...")
The input consist of a number of operations. Each operations (erase, insert and S) are separated by a newline character ('\n').
Operation insert: following by a non-negative integer (insert position) and a non-negative integer (insert value, 0<=insert value<65535). Insert a node at insert position with insert value.
Operation erase: following by a non-negative integer (erase position). Erase a node at erase position.
Operation show: print the value of all nodes in the list. Print a whitespace character (' ') after printing the value of nodes. If the list is empty, do not print anything.
如果不知道operator<<如何實作，可以看 main.cpp
Implement List (constructor, destructor, erase and insert), operator<<(std::ostream &,const List &lst).