To put this in context, I'm looking at question 23 of Quiz 0 Fall 2014 (http://cdn.cs50.net/2014/fall/quizzes/0/key0.pdf)
(Don't click the link if you don't want to see the solution! The question is how to implement strlen() )
My solution is almost identical to the correct solution, the main difference being I have
while (ptr != NULL) instead of
while (s[n] != '\0').
Is there a difference between using NULL and using the 'null-character'?