leetcode_96_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 int numTrees(int n) {
		int arr[]=new int[n+1];
		int len=0;
		Arrays.fill(arr, 0);
		arr[0]=1;
		arr[1]=1;
		for(int i=2;i<=n;i++)
		{
			len=i-1;
			for(int j=0;j<i;j++)
			{
				arr[i]+=arr[j]*arr[len-j];
			}
		}
		return arr[n];
    }