HDU How Many Trees?

原题链接:

思路:卡特兰数，建议百度一下，挺实用的一个组合数。还有一点这是一道大数题，为了偷懒，我拿出了我的java大数类

import java.math.BigInteger;import java.util.Scanner;/** * Created by ZUOYANBIN1 on 2017/9/18. */public class Main {    static int n = 0;    static void solve(){        BigInteger ans = new BigInteger("1");        for(int i = 1; i <= n; i++){            ans = ans.multiply(new BigInteger(String.valueOf(4 * i - 2)))                    .divide(new BigInteger(String.valueOf(i + 1)));        }        System.out.println(ans);    }    public static void main(String[] args){        Scanner in = new Scanner(System.in);        while(in.hasNext()){            n = in.nextInt();            solve();        }    }}