It is nothing but counting number of unique structure possible
If we are talking about unlabled binary tree then there is formula for it $\frac{1}{n+1}^{2n}\mathbb{C}_{n}$
If we take n= 2 we get 2 unique structure
For n=3 fine unique structure
And. Labled binary search tree would be =$\frac{1}{n+1}^{2n}\mathbb{C}_{n}*n!$