Please help me, i don't understand what is wrong with the code below


import java.math.BigInteger;
import java.io.*;
import java.util.*;

public class LuckyTickets
{


    public static void main(String[] args)
    {

        int N,S;
        BigInteger F[][] = new BigInteger[51][];

        for(int i = 0; i <= 50; i++)
            F[i]= new BigInteger[1001];

        Scanner scanner= new Scanner(System.in);
        try
        {
            N= scanner.nextInt();
            S= scanner.nextInt();


            for(int j= 0; j <= N; j++)
                for(int i= 0; i <= S; i++)
                    F[j][i]= new BigInteger("0");

            for(int j= 1; j <= N; j++)
                F[j][0] = BigInteger.ONE;

            for(int i= 1; i < 10; i++)
                F[1][i] = BigInteger.ONE;

            for(int i= 2; i <= N; i++)
                for(int k= 1; k <= S; k++)
                {
                    for(int j= 0; j <= 9 && j <= k; j++)
                        F[i][k]= F[i][k].add(F[i-1][k-j]);
                }

            // there are F[N][S/2] * F[N][S/2] different tickets
            System.out.println(F[N][S/2].multiply(F[N][S/2]));

//            for(int j= 0; j <= N; j++){
//                for(int i= 0; i <= S; i++)
//                    System.out.print(F[j][i]+ " ");
//                System.out.println("");
//            }

        }
        catch(InputMismatchException e)
        {
            System.out.println("Mismatch exception " + e);
        }

    }
}

you don't need 2-dimension F[][], just only using one-dimension BigInteger array (in your case, F[]) is enough.

for(int j= 0; j <= 9 && j <= k; j++)  change this to for(int j= 1; j <= 9 && j <= k; j++)

check answer '0' at the begining for two cases:  S is odd or S > 18 * n

second dimension of F is [s + 1] instead of 1001 which kills memory..

I got accepted using Java biginteger as well, and I create the array of below:

BigInteger []f = new BigInteger[s + 1];

That is it!