c++ problem.

When I use the Ackermann function in my cxx code, I am not sure about the third condition. I don't know why some codes online said like below. why "retval=m+n;" instead of using nested calculate function?

n 1 m, n J A(m 1, 1) if m 0 and n 0 A (m -1, A(m, n 1)) if m 0 and n 0