4 задача. нахождение произведения

#include < iostream>
#include <vector>

using namespace std;


vector<int> multiplication(const vector<int>& array, const int& lenght)
{
    if (lenght % 2 == 0)
    {
        vector<int> output(lenght * 2 - 1,1);
        for (int i = 0; i < lenght; i++)
        {
            output[lenght - 1 + i] = array[i];
        }
        for (int i = lenght - 2; i >= 0; i--)
        {
            output[i] = output[i * 2 + 1] *output[i * 2 + 2];
        }
        return output;
    }
    else
    {
        vector<int> output((lenght + 1) * 2 - 1,1);
        for (int i = 0; i < lenght; i++)
        {
            output[lenght + i] = array[i];
        }
        for (int i = lenght - 1; i >= 0; i--)
        {
            output[i] = output[i * 2 + 1] * output[i * 2 + 2];
        }
        return output;
    }
}

int main()
{
vector<int>     array;
    cin >> number;
    for (int i = 0; i < number; i++)
    {
        cin >> x;
        array.push_back(x);
    }

vector<int> a = multiplication(array, array.size());
    cout << endl << endl;
    cout << a[0] % 12345;
}