source: https://bruceoutdoors.wordpress.com/2017/03/15/the-programmers-guide-to-fft-part-1-dft/

polynomial-multiply.cpp

#include <iostream>     
#include <vector>

using namespace std;

typedef vector<int> ivec;

ivec polynomialMultiply(const ivec &a, const ivec &b) 
{
    int N = 2 * a.size() - 1;
    ivec c(N, 0);
    
    for (int j = 0; j < N; ++j) {
        int p = j >= a.size() ? j - a.size() + 1 : 0; 
        for (int k = p; k <= j - p; ++k) {
            c[j] += a[k] * b[j - k];
        }
    }
    
    return c;
}

int main()
{
    ivec a = {  6, 7, -10,  9 };
    ivec b = { -2, 0,   4, -5 };
    
    ivec c = polynomialMultiply(a, b);
    
    for (const auto &t : c)  cout << t << ' ';
    cout << endl;
}