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

 public class Solution {

     public int numTrees(int n) {

         if(n==0||n==1)

         return 1;

         int res=0;

         for(int i=1;i<=n;i++)

         {

             res+=numTrees(i-1)*numTrees(n-i);

         }

         return res;

     }

 }