HDU 1130 How Many Trees

HDU 1130 How Many Trees?
Problem DescriptionA binary search tree is a binary tree with root k such that any node v reachable from its left has label (v) <label (k) and any node w reachable from its right has label (w) > label (k). It is a search structure which can find a node with label x in O(n log n) average time, where n is the size of the tree (number of vertices). 

Given a number n, can you tell how many different binary search trees may be constructed with a set of numbers of size n such that each element of the set will be associated to the label of exactly one node in a binary search tree? 
 InputThe input will contain a number 1 <= i <= 100 per line representing the number of elements of the set.
 OutputYou have to print a line in the output for each entry with the answer to the previous question.
 Sample Input

123

125

import java.util.*;import java.math.*;public class Main{    public static void main(String[] args)     {        int n;        Scanner in=new Scanner(System.in);        BigInteger one=BigInteger.ONE;        BigInteger four=new BigInteger("4");        BigInteger two=new BigInteger("2");        BigInteger st=null;        BigInteger ans=null;        while(in.hasNextInt())        {            n=in.nextInt();            ans=BigInteger.ONE;            for(int i=2;i<=n;i++)            {                st=new BigInteger(""+i);                ans=ans.multiply(st.multiply(four).subtract(two)).divide(st.add(one));            }            System.out.println(ans);        }    }}