Not sure, but I think may be the fifth(last part of string) %d holds the 'end of the string character (or so called null termination) "\0"'.

I tested it. The code works with or without the very last %d. It was probably placed there by mistake.

I've never even heard of snprintf
But I must admit that the solution is pretty to read.
Though I must ask about the fifth "%d" in the last part of the string. Why is that there?

I was pleasantly surprised to see that the solution I thought would be considered an inefficient "naive" approach seems to be a pretty popular one, since I seem to be unfamiliar with advanced string manipulation/creation in C++.
Mine is pretty much the same, just with some minor formatting differences