FactorialSolver.java

import java.util.*;

public class FactorialSolver 
{
    private static HashMap<Integer, Integer> factorials;
    
    public static int factorial(int n) throws Exception {
        if (factorials == null) {
            factorials = new HashMap<Integer, Integer>();
            factorials.put(0, 1);
        }
        
        if (n < 0) {
            throw new Exception();
        }
        
        if (factorials.containsKey(n)) {
            return factorials.get(n);
        }
        else {
            int res = factorial(n - 1) * n;
            factorials.put(n, res);
            return res;
        }
    }
}