It's necessary to qualify a verified certificate. Given that your code does not need to be submitted, the video is the proof that you actually completed your project and is your opportunity to demonstrate it.
The function signature has changed. From the CS50 Programmer's Manual
char *get_string (const char *format, ...);
Prompts user for a line of text from standard input and returns it as
a string (char *), sans trailing line ending. Supports CR (\r), LF
(\n), and CRLF (\r\n) as line endings. Stores string on heap, but
The only way I could think of is the history API.
It's often used by single page applications (SPAs) to implement routing, so if you use browser controls to reload or move back and forth, you get more or less to the same state as before.
The current versions (Fall 2015) of the lecture videos are available on youtube at the following link: https://www.youtube.com/playlist?list=PLhQjrBD2T383Xfn0zECHrOTpfOSlptPAB
However I don't know if those match the pset(s), etc. of what's currently available on the edX CS50 site.