Given n, how many structurally unique BST‘s (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST‘s.

   1         3     3      2      1    \       /     /      / \           3     2     1      1   3      2    /     /       \                    2     1         2                 3

 1 public class Solution { 2     public int numTrees(int n) { 3         if(n==0||n==1) 4         return 1; 5         int res=0; 6         for(int i=1;i<=n;i++) 7         { 8             res+=numTrees(i-1)*numTrees(n-i); 9         }10         return res;11     }12 }

Unique Binary Search Trees In JAVA