Question

I am working on an assignment where I have to implement a recursive way to calculate the combination of two numbers. For example, 5C3 would be 10. That is, there are 10 combinations of 3 objects out of 5 total. However, I would like to implement a way to use the BigInteger class so I can calculate larger combinations, such is 2400 pick 3. For some reason, my code still returns a negative number, much like the behavior if it were to be regular integers. I’ve included my code below. Could someone please tell me where I am going wrong?

import java.math.BigInteger;
public class Combination {

public static BigInteger[][] memo = new BigInteger[3000][3000];

public static BigInteger choose(BigInteger n, BigInteger k) {

    if (n.intValue() == 0 && k.intValue() > 0) {
        return BigInteger.ZERO;
    } else if (k.intValue() == 0 && n.intValue() >= 0) {
        return BigInteger.ONE;
    } else if (memo[n.intValue()][k.intValue()] != null) {
        return memo[n.intValue()][k.intValue()];
    } else {
        memo[n.intValue()][k.intValue()] = choose(n.subtract(BigInteger.ONE), k.subtract(BigInteger.ONE)).add(choose(n.subtract(BigInteger.ONE), k));
    }
    return memo[n.intValue()][k.intValue()];

}

public static void main(String args[]) {

    if (args.length < 1) {
        System.out.println("Usage: java Combination <N> <K>");
        System.exit(0);
    }

    int H = Integer.parseInt(args[0]);
    int R = Integer.parseInt(args[1]);

    BigInteger N = BigInteger.valueOf(H);
    BigInteger K = BigInteger.valueOf(R);

    System.out.println(choose(N, K).intValue());

}

}

Answer 1

System.out.println(choose(N, K).intValue());

should be

 System.out.println(choose(N, K).toString());