Unique Binary Search Trees

Problem:

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

Analysis:

Refer http://blog.joysword.com/en/posts/2014/02/unique_binary_search_trees/.

Solution:

 public class Solution {  
   public int numTrees(int n) {  
     if(n==0)  
      return 1;  
     if(n==1)  
      return 1;  
     int num=0;  
       for(int j=1;j<=n;j++)  
       {  
         num+=numTrees(j-1)*numTrees(n-j);  
       }  
      return num;  
   }  
 }