F(n)=∑(F(n- 1-k)* Fk)； Where k is from 0 to n- 1.

It is known that F0= 1,

F 1=F0*F0= 1

F2=F 1*F0+F0*F 1=2

F3 = F2 * F0+f 1 * f 1+F0 * F2 = 5

……

It is proved that for n data, I only look at the order of the first data entering and leaving the stack:

The first data can contain 0, 1, 2…n- 1 data in and out of the stack.

Accordingly, after the first data is popped up, there are still n- 1, n-2...2, 1, 0 data to be popped up and put on the stack.

According to the multiplication principle in combinatorial mathematics, it is necessary to multiply the species before and after the first data is out of the stack.

According to addition principle, it is necessary to add all n paths of the first data in and out of the stack.

So we get the recursive formula, but it seems difficult to find a formula to calculate Fn directly.