Unique Binary Search Trees

Unique Binary Search Trees

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 result = 0;
　　　　for(int i = 0; i < n; i++)
　　　　　　result += numTrees(i)*numTrees(n-i-1);
　　　　return result;
　　}
}