Iterative derangements / subfactorials using BigDecimals and BigIntegers in Java

Derangement

	public BigInteger derangement(int num) {
		if (num == 0) {
			return BigInteger.ONE;
		}

		BigDecimal sum = BigDecimal.ZERO;
		for (int i = 0; i <= num; i++) {
			BigDecimal fact = new BigDecimal(factorial(i));
			BigDecimal toAdd = BigDecimal.valueOf(Math.pow(-1, i)).divide(fact, MathContext.DECIMAL128);
			sum = sum.add(toAdd);
		}

		BigDecimal answer = new BigDecimal(factorial(num)).multiply(sum, MathContext.DECIMAL128);
		return answer.toBigInteger();
	}