Recursion Tracing Exercise: Recursive function nested calls

Consider the following code: public long foo(long x) { if(x == 1 || x == 3) return x; else return x * foo(x-1); } Assuming no possibility of integer overflow, what will be returned by the following call to foo? foo(foo(3)+foo(4));

(15!)/(2!)

3!+4!
(7!)!
(3!+4!)!
15

This function computes factorial, except that it leaves out 2 from the series of multiplications.

So, foo(3)+foo(4) = 15.