Extra point No. 1: Runaway pointer (2 points in the final grade that can
apply to either programming assignment or exam, Due: Oct. 9, 11:59pm)

Pointers in a linked list can easily cause problems. A buggy linked list 
program may create a loop in a linked list: the 'next' pointer in the tail 
of the list points back to one of the items in the list. In this challenge,
you will implement a routine 'int testloop (listnode *head)' 
to check whether the linked list 'head' contains a loop. If the linked list contains
a loop, testloop returns the size of the loop (the number of elements in the loop). 
Otherwise, testloop returns 0. The following are the requirements for the routine:

(1) O(1) memory complexity.
(2) O(N) time complexity, where N is the length of the linked list.
(3) The routine should not modify the linked list.

Briefly explain (informal proof) why your code has O(1) memory complexity
and O(N) time complexity. Put the analysis as comment in your program. 
(1.5 point for the code and 0.5 point for the analysis).

Three files are given: main.cpp, create.o, and extra1.x. 
You can implement the testloop routine in main.cpp and compile
on linprog with 'g++ -o extra1.x main.cpp create.o'.