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

Java

public class Solution {
    public int numTrees(int n) {
        int[] num = new int[n + 1];

        num[0] = 1;
        num[1] = 1;

        for(int i = 2; i <= n; i++) {
            for(int j = 0; j < i; j++) {
                num[i] += num[j] * num[i - 1 - j];
            }
        }
        return num[n];
    }
}