Question

I want to check whether two vectors have any elements in common. What’s wrong with this syntax?

// Check whether the current list and the input l2 share any nodes or not 
bool shared(const VectorList< NODETYPE > &l2); 

template< typename NODETYPE > //SHARED
bool VectorList< NODETYPE>::shared(const VectorList< NODETYPE > &l2)
{

    for(int i = 0; i < (int)vList.size(); i++)
      {
        for (int j = i; j < (int)l2.size() ; j++)
            {
                    if (vList[i] == l2[j])
                    {
                        return(1);
                    }
            }
      }

    return(0);

}

Answer 1

Assuming you have implemented VectorList as (similar to) a standard container, I’d consider writing (see find_first_of):

template<typename T>
bool VectorList<T>::shared(const VectorList<T> &l2) const // <-- NOTE added const
{
    return end() != std::find_first_of(
        begin(), end(), 
        l2.begin(), l2.end());
}

Note that the (worst-case) runtime complexity will still be quadratic (or O(n*m))