Combination.java

import java.util.HashMap;
import java.util.Map;

public class Combination {

    private Map<Integer,Long> factorialMap = new HashMap<Integer,Long>();

    public Long getFactorial(int number) {
        Long val = factorialMap.get(number);
        if(val != null) {
            return val;
        } else {
            val = getFactorialRecursive(number);
            factorialMap.put(number, val);
            return val;
        }
    }

    public Long getFactorialRecursive(int number) {
        if(number == 1 || number == 0) {
            return 1L;
        } else {
            return number * getFactorialRecursive(number-1);
        }
    }

    public Long combination(int fromVal, int chooseVal) {
        return getFactorial(fromVal)/(getFactorial(chooseVal)*getFactorial(fromVal-chooseVal));
    }


    public static void main(String[] args) {
        Combination comb = new Combination();
        for(int i = 0; i < 20; i++) {
            System.out.print(20 +" chose " + i + " = ");
            System.out.println(comb.combination(20,i));
        }

    }
}