Leetcode-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

Have you met this question in a real interview?

Solution:

 1 public class Solution {
 2     public int numTrees(int n) {
 3         int[] d = new int[n+1];
 4         Arrays.fill(d,0);
 5         d[0] = 1;
 6         for (int i=1;i<=n;i++)
 7             for (int j=1;j<=i;j++)
 8                 d[i] += d[j-1]*d[i-j];
 9         return d[n];
10     }
11 }