URAL 1036 Lucky Tickets

URAL_1036

    用f[i][j]表示递推到第i位时数字和为j的方案数，最后用下乘法原理f[N][S/2]*f[N][S/2]就是最后结果。

    由于结果比较大，所以需要高精度。

import java.math.BigInteger;
import java.util.Scanner;

public class Main {
    static int N, S;
    static BigInteger[][] f = new BigInteger[60][510];
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        while(cin.hasNext())
        {
            N = cin.nextInt();
            S = cin.nextInt();
            if(S % 2 == 1)
                System.out.println("0");
            else
                solve();
        }
    }
    static void solve()
    {
        int i, j, k;
        BigInteger sum;
        S /= 2;
        for(i = 1; i <= S; i ++)
            f[0][i] = new BigInteger("0");
        f[0][0] = new BigInteger("1");
        for(i = 1; i <= N; i ++)
        {
            sum = new BigInteger("0");
            k = 0;
            for(j = 0; j <= S; j ++)
            {
                if(j - k > 9)
                {
                    sum = sum.add(f[i - 1][k].negate());
                    ++ k;
                }
                sum = sum.add(f[i - 1][j]);
                f[i][j] = sum;
            }
        }
        System.out.println(f[N][S].multiply(f[N][S]));
    }
}