Unique Binary Search Trees In JAVA

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 }