Reverse Polish Notation

It can also be 2 9 7 - / for it to work.

If you want to visualize it:

Input	Operation	Stack
2	Push Back	[2]
9	Push Back	[2,9]
7	Push Back	[2,9,7]
-	Subtract	[2,(9-7)]
-	Push Result	[2,2]
/	Divide	[(2/2)]
/	Push Result	[1]

Likewise, I think yours is still correct.

Input	Operation	Stack
9	Push Back	[9]
7	Push Back	[9,7]
-	Subtract	[9-7]
-	Push Result	[2]
2	Push Back	[2,2]
/	Divide	[2/2]
/	Push Result	[1]