Computes the largest product of N numbers in a of a sequence of N or more numbers.

largest_product_of_n.cpp

int largest_product_of_n(std::vector<int> values, std::size_t n)
{
	if (values.size() < n)
	{
		throw std::invalid_argument("Sequence size must at least n!");
	}
	
	std::sort(values.begin(), values.end());
	
	int positive_product = 1;
	
	for (std::size_t count = 0, i = values.size() - 1; count < n; ++count, --i)
	{
		positive_product *= values[i];
	}
	
	if (values[1] >= 0) return positive_product;
	
	int negative_product = 1;
	
	int last_negative = 0;
	
	std::size_t i = 0;
	
	for ( ; i < n; ++i)
	{
		if (values[i] >= 0) break;
		
		if (last_negative == 0)
		{
			last_negative = values[i];
		}
		
		else
		{
			negative_product *= last_negative * values[i];
			
			last_negative = 0;
		}
	}
	
	if (last_negative != 0) --i;
	
	for (std::size_t j = values.size() - 1; i < n; ++i, --j)
	{
		negative_product *= values[j];
	}
	
	return std::max(positive_product, negative_product);
}