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:

public class Solution {
    public int numTrees(int n) {
        int[] d = new int[n+1];
        Arrays.fill(d,0);
        d[0] = 1;
        for (int i=1;i<=n;i++)
            for (int j=1;j<=i;j++)
                d[i] += d[j-1]*d[i-j];
        return d[n];
    }
}