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 <= 1)  return 1;                int sum=0;        int left,right,root;                for(root=1; root<=n; root++){            left= numTrees(root-1);            right = numTrees(n-root);            sum += left * right;        }                return sum;    }    }